Covalent organicframeworks (COFs) are porous materials with highsurface areas, making them interesting for a large variety of applicationsincluding energy storage, gas separation, photocatalysis, and chemicalsensing. Structural variation plays an important role in tuning COFproperties. Next to the type of the building block core, bonding directionality,and linking chemistry, substitution of building blocks provides anotherlevel of synthetic control. Thorough characterization and comparisonof various substitution patterns is relevant for the molecular engineeringof COFs via rational design. To this end, we have systematically synthesizedand characterized multiple combinations of several methylated andnon-methylated building blocks to obtain a series of imine-based COFs.This includes the experimental assignment of the COF structure bysolid-state NMR. By comparing the properties of all COFs, the followingtrends were found: (1) upon methylation of the aldehyde nodes, COFsshow increased Brunauer–Emmett–Teller surface areas,reduced pore collapse, blue-shifted absorbance spectra, and ∼0.2eV increases in their optical band gaps. (2) COFs with dimethylatedamine linkers show a lower porosity. (3) In tetramethylated aminelinkers, the COF porosity even further decreases, the absorbance spectraare clearly red-shifted, and smaller optical band gaps are obtained.Our study shows that methyl substitution patterns on COF buildingblocks are a handle to control the UV absorbance of the resultingframeworks. COFs have been reported as good photocatalystsfor such clean and green transformations.8 The first COFs were linked by boronic esters, which were susceptibleto hydrolysis. This has been improved by using imine-linked COFs oreven more stable β-ketoenamine COFs.10 In the latter case, imine bonds are formed first, which then tautomerizeirreversibly into the β-ketoenamine linkages. The irreversibletautomerization results in lower crystallinity and surface area comparedto their imine equivalents, which is the reason why imine COFs arewidely used.4 Imine COFs can be synthesizedin a condensation reaction from aldehyde and amine building blocksunder acidic catalysis, which is also known as Schiff base chemistry.13 Vacuum drying has often been used but was identifiedto induce partial pore collapse due to capillary forces.14 Capillary forces increase with decreasing poresizes and COFs, which usually have pore sizes around a few nanometers,are therefore subjected to strong capillary forces. To overcome porecollapse, milder activation methods such as supercritical CO2 drying14 or washing with ultralow surfacetension solvents prior to drying15 weresuggested to keep the framework intact. Feriante et al.14 and Zhu and Verduzco15 both reported a methoxy-functionalized, imine-based COF, which wasless prone to pore collapse compared to the initial unfunctionalizedCOF. Recently, we have found that COFs consisting of 2,4,6-trimethylbenzene-1,3,5-carbaldehyde(Me3TFB) lead to more robust frameworks compared to non-methylated1,3,5-benzenetricarbaldehyde (TFB).16Porous materials area class of solids with pore sizes of a fewto several hundred nanometers. These pores can be classified intomacro- (>50 nm), meso- (2–50 nm), and micropores (<2nm).17 reportedon how an acidic catalyst influences the porosity of obtained COFmaterials. Namely, they studied the effect of amine substitution onβ-ketoenamine-linked COF properties. They investigated the porositybased on differences in hydrogen bonding in an intermediate to beable to predict the porosity of the resulting COFs. These β-ketoenamine-linkedCOFs were synthesized via a mechanochemical synthesis of 1,3,5-triformylphloroglucinol(Tp) with several functionalized phenylenediamine (PA) and benzidine(BD) linkers, catalyzed by different toluenesulfonic acids or hydrogenchloride. During this process, the diamine first forms an acid-diaminesalt with the toluenesulfonic acids, which then further reacts withTp to the final COF. The hydrogen-bond distance of the acid-diaminesalt was found to affect the COF porosity. While the hydrogen-bonddistance was tuned by the choice of toluenesulfonic acid, no trendbetween the Brunauer–Emmett–Teller (BET) area and theirmethylated amine linker was observed. Additionally, the applied mechanochemicalsynthesis—especially the intermediate acid-diamine salt—differsfrom COFs synthesized in solution. In another study, Singh et al.9 reported on the possibility to fine-tune theoptical band gap by changing the number of hydroxyl groups on theTFB aldehyde node by varying the push–pull electronic effect.The authors found that the more pronounced push–pull effectleads to the lower resulting band gap. Combined with the earlier reportedincreased stability of methyl-substituted COFs,16 these two studies motivated us to systematically investigatethe effect of methyl groups on the porosity and UV absorbance of imine-basedCOFs. To this end and given their pore stability and utilization potentialof their UV absorbance, we decided to introduce an increasing numberof methyl groups, this time also in the amine linkers. Such thoroughcomparison of similar, yet different structures, enables a fundamentalunderstanding, which can be employed to rationally design COF materialswith tailor-made properties. By expanding our methylated imine-basedCOF library and by thoroughly characterizing, including detailed solid-stateNMR (ssNMR) measurements as well as experimental band gap analyses,we investigate the influence of methylation of aldehyde nodes andamine linkers on the resulting COF crystallinity, BET surface areas,the tendency for pore collapse, and the UV absorbance of the COFs.While COF stability is essential for virtually all applications,other properties, like selectivity and catalytical performance, requiretailor-made approaches. Identification of structure–propertyrelationships of COFs would facilitate the process of controllingsuch properties. Karak et al.16 All solventsand glacial acetic acid (AR) were purchased from commercial sourcesand used without further purifications.1,4-Phenylenediamine (>98% (GC) (T)), 2,5-dimethyl-1,4-phenylenediamine(>98%), and o-tolidine (>98%) were purchased from TCI EuropeN.V.2,3,5,6-Tetramethyl-1,4-phenylenediamine (>98% (GC) (T)) was purchasedfrom Sigma-Aldrich, benzidine (98%) was purchased form Abcr, 2,3,5,6-tetramethylbenzidine(99%) was purchased from ChemPUR, and all chemicals were used withoutfurther purification. 1,3,5-Benzenetricarboxaldehyde (96%) was purchasedfrom TCI Europe N.V. and Fluorochem. Mesitylene was purchased from Fisher Scientific, and 1,4-dioxane (99%) was purchasedfrom Acros Organics B.V.B.A. 2,5,6-Trimethyl-1,3,5-benzenetricarboxaldehydewas synthesized before.1H and 13C NMR spectra were recorded on aBruker AVANCE III NMR spectrometer at 400 and 100 MHz, respectively.The spectra were referenced with respect to the deuterated solvents. 1H and 13C{1H} cross-polarization magic angle spinning (CPMAS)ssNMR spectra were recorded on a Bruker AVANCE III HD spectrometerat 700.13 MHz (16.4 T) and 176 MHz, respectively. ssNMR samples werepacked into 4 mm zirconia rotors and spun at MAS frequencies of 11and 14 kHz at 298 K. The 13C CPMAS spectra were recordedby using a CP pulse sequence. SPINAL64 decoupling was applied on protonswith a decoupling strength of 104 kHz. The 13C CPMAS spectrawere obtained with a recycle delay of 3 s, and the strength of theCP contact pulses of 0.08 kHz related to a contact time of 3 ms, unlessstated differently. To obtain the CP build-up curves, the CP contactedpulses were varied between 0.04 and 25 kHz and related to contacttimes between 6.5 ms and 10 μs. The 13C ssNMR spectrawere referenced with respect to adamantane . The spectra were analyzed using MestReNova (version 14.1.0)and ssNake,18 which allows a fitting ofthe signals, opening up the possibility to integrate the signals.These relative peak integrals can be plotted against the contact timeto obtain CP build-up curves.–1.Fourier-transform infrared (FT-IR)spectra were obtained on a BrukerTensor 27 spectrometer with an attenuated total reflection accessorycalled platinum. The samples were applied as powder on top of thecrystal. 64 scans were performed with a resolution of 4 cmPowder X-ray diffraction measurements were performed witha PhilipsX’pert-PRO at 40 kV and 40 mA from 4 to 40°  and from 1.5 to 10°. X-rayswere generated by a Cu anode Kα (1.54 λ) radiation. Thegoniometer radius was 240 mm, the soller slits were 2.3°, andthe receiving slit was 0.1 mm in width. Pawley refinement was carriedout in Topas (version 5) with a CuKa5_Berger emission profile. Theparameters “Zero point error”, “Cry Size L”, and “Strain L” were allowed to refine.Nitrogenadsorption–desorption measurements were performedon a MicroActive for Tristar II Plus 2.01 at 77.350 K. Before themeasurement, the samples were outgassed at 120 °C overnight.Surface areas were calculated from the adsorption data using BET methodsand Rouquerol criteria. The pore-size distribution curves were obtainedfrom the adsorption branches using the two different density-functionaltheory (DFT) methods:(1) N2—cylindrical pores—oxidesurface.2@77K.(2) HS-2D-NLDFT, carbon cylindrical pores (ZTC), andNThe second method is carbon based and thereforechemically closer,but the first method can show the pore size distribution for someCOFs down to smaller diameters. An optimum between Goodness of Fitand smoothness of the pore size distribution was aimed for. The averageof three different COF batches was used to determine the BET surfaceareas.The retained surface area was calculated based on Eq Diffuse reflectance spectrawere recorded on a Cary4000 spectrometerwith an integrating sphere from Agilent. An empty sample holder wasused for calibration of 100% light intensity at the detector and ablack surface for 0% light intensity.Thermogravimetric analysiswas performed on a PerkinElmer STA 6000.The sample was heated to 30 °C, and this temperature was maintainedfor 1 min, before the sample was heated with 10 °C/min to 700°C in a nitrogen atmosphere with a flow rate of 20 ml/min. Thethermal stability was determined at the point where 95% of the sampleswere still retained.Origin2020b (64 bit) version 9.7.5.184was used to analyze, plot,and fit all data.20 The PBE functional based on the generalized gradient approximationwas chosen to account for the exchange–correlation energy.21 A plane-wave basis set in combination with theprojected augmented wave method was used to describe the valence electronsand the valence–core interactions, respectively.22 The kinetic energy cut-off of the plane wavebasis set was set to 500 eV. Gaussian smearing of the population ofpartial occupancies with a width of 0.05 eV was used during iterativediagonalization of the Kohn–Sham Hamiltonian. The thresholdfor energy convergence for each iteration was set to 10–5 eV.The crystal structures were fully relaxed, and geometries were assumedto be converged when forces on each atom were less than 0.05 eV/Å.The Brillouin zone integration and k-point samplingwere done with a γ centered 1*1*8 and 2*2*4 grid points forthe eclipsed and staggered unit cells, respectively. The van der Waalsinteractions were included by using Grimme’s DFT-D3(BJ) methodas implemented in VASP.23 Geometries werevisualized, and simulated powder-X-ray diffraction (PXRD) patternswere obtained by using VESTA (version 3.4.8).24 Coordinates of all crystal structures are provided at the end ofthis article.All DFT calculations were performed by usingthe Vienna ab initiosimulation package .25The COF synthesis isbased on a modified procedure of Smith et al.The aldehyde (1 equiv) and amine monomers (1.5 equiv) wereadded to a 50 mL round bottom flask, together with a stirring rod,and were dissolved in 4:1 v/v 1,4-dioxane:mesitylene mixture. Themixture was heated to 70 °C for 5 min to ensure dissolution.After cooling the mixture to approximately 40 °C, water and glacialacetic acid  were added.The reaction mixture was stirred at 70 °C for 3 days. Afterward,the reaction was cooled to RT, and the precipitate was collected viaBüchner filtration. The solid was dispersed in dimethylformamide(DMF), stirred at 90 °C for 30 min, and collected via Büchnerfiltration. These steps were repeated with DMF ,ethanol , acetone , and hexane. After the final Büchner filtration, theCOFs were divided over two petri dishes and covered with tin foilfor drying. The COFs were dried overnight at 120 °C, either ina regular oven or in a vacuum oven. After drying, the COFs were keptin the glovebox for storage.3TFB) and non-methylated (TFB) aldehyde nodes and (b) varyingnumbers of methyl groups on two different amine linkers  whose structuresare presented in 3TFB–Me4PA, Me3TFB–Me2BD, Me3TFB–Me4BD, TFB–Me2PA, and TFB–Me4BD. Keeping the syntheticand experimental conditions similar for all COFs within this and ourprevious study—in which we compared TFB–PA and TFB–BDto Me3TFB–PA and Me3TFB–BD foracid vapor sensing16—we also synthesizedthe COFs that have been published before , completing the matrix in Tables S1–S8).We synthesized a full range of COFs with (a) methylated . One batch was dried at 120°C overnight in an air-ventilated oven, and the other one wasdried at 120 °C overnight in a vacuum oven.In a classicalSchiff base condensation reaction, 2 equiv of aldehyde node and 3equiv of amine linker were reacted at 70 °C for 3 days in 1,4-dioxane:mesitylene1:4 v/v at atmospheric pressure. While reactions with unmethylatedamines were carried out with additional water to enhance the dynamiccovalent chemistry (DCC), methyl-containing amines were reacted onlywith acetic acid acting as catalyst. The additional water can notonly enhance error-correction via DCC, but in the case of methylatedamines, the equilibrium was also shifted toward the starting materialsas indicated by FT-IR spectroscopy, which showed a higher intensityfor the C=O stretch of the aldehyde around ∼1690 cm–1 , FT-IR spectroscopy confirmedthe successful formation of imine bonds. All COF spectra show an attenuatedC=O stretch of the aldehyde around ∼1690 cm–1, which is likely due to unreacted side groups at the outside ofthe 2D polymeric sheet.32 These relative intensities vary depending on the number of methylgroups attached to the amine linker. The more methyl groups are attached,the stronger is the residual aldehyde vibration in the spectrum. Therepeatability for TFB–Me4PA is less good comparedto the other COFs of this series. To shed light on the full COF series,this sample was still included in the further analysis. The lowerrepeatability is expressed by the larger error margins.First, basedon new bands that appeared near 1620 cm4BD, show diffraction peaks . Some diffraction peaksare low in intensity and broad, but together with the BET surfacearea determination, we concluded that the obtained structures areCOFs. However, based on its PXRD and BET characteristics, TFB–Me4BD is considered a COF-like porous organic polymer.Thecrystallinity of the COFs was confirmed by PXRD analysis. Thecrystallinity and the BET surface areas of COFs are typically closelyrelated, so both measurements were taken into account together forthe characterization of the materials. All diffractograms, exceptfor TFB–MeP6/m, whichcorresponds to an eclipsed stacking structure. The unit cell dimensionsand Rwp and Rp values are given in 33 For TFB–Me2BD, compared to literature, a largerinterlayer stacking distance (3.73 ± 0.02 vs 3.4 Å) andlower Rwp and Rp values were obtained.28 For TFB–Me4PA, the refinement matched better to the space group P1 with lattice parameters of a = 22.80± 0.46 Å, b = 22.21 ± 0.26 Å, c = 6.74 ± 0.24 Å, α = 89.5 ± 1.21°,β = 91.1 ± 3.82°, and γ = 59.1 ± 1.10°with Rwp = 3.42 ± 1.24% and Rp = 2.23 ± 0.60%. For TFB–Me4BD, no Pawley refinement was conducted, because of the non-existingcrystallinity. The Pawley-refined PXRD patterns were compared to thesimulated diffraction patterns of optimized crystal structures obtainedfrom DFT calculations (coordinates in Supporting Information). To compare experimental COF with simulated PXRDpatterns, Zhang et al. suggested to use simulated patterns of statisticalCOF models, consisting of more than one single stacking order, toaccurately model the PXRD diffractions of COFs.34 To obtain insights into the effect of stacking on the characteristicsof PXRD patterns, the two extreme models “eclipsed”and “staggered” were simulated. Small local disorderswere neglected to reduce computational complexity. Such small deviationsin peak position and peak broadening can also be observed in our materials.It was therefore hypothesized that the small deviations in peak positionbetween the experimental and the DFT-computed PXRD patterns are relatedto stacking disorder. All COFs—except for TFB–Me4PA—crystallize predominantly in an eclipsed stackingconformation including some stacking disorder. The deviations betweenthe computed unit cell dimensions and the Pawley-refined values arebelow 5%, which is within the expected accuracy of the DFT calculations,further supporting the formation of eclipsed COFs. Only the deviationin interlayer stackings of Me3TFB–Me4BD is slightly higher (6.8%). The diffraction patterns  and unit cell dimensions of TFB–Me4PA exhibit a better match with the staggered model as alreadyindicated by the different space group.Pawley refinement was performed in triplicate, with the PXRD diffractogramusing the space group Upon increasing the number of methyl groupsin the amine linker,the crystallinity decreases. This can be rationalized by a less reactivetetramethyldiamine due to steric hindrance. The more unreacted aldehydegroups remain at the outside of the 2D polymeric sheets, the higheris the intensity of the C=O stretchin the FT-IR spectra. For aldehyde nodes, the crystallinity is largerwhen using methylated building blocks.N,N-dimethylformamid (DMF), COFs were immersed for 5 days into the respectivesolvents, and upon re-isolation and drying at 120 °C overnight,the PXRD patterns were recorded again. To investigate the chemicalstability of the novel COFs, representative samples  were chosen: Me3TFB–PA, TFB–Me4PA, Me3TFB–Me4PA, and Me3TFB–BD . Wang et al.35 and Daugherty et al.36 already studiedthe chemical stability of TFB–PA and TFB–BD, respectively.Me3TFB–PA, TFB–Me4PA, Me3TFB–Me4PA, and Me3TFB–BD werefound to be stable in 1 M NaOH and DMF. All tested COFs are chemicallystable in DCM except for Me3TFB–BD, which is onlypartially stable in DCM. All COFs are stable in water, except forMe3TFB–Me4PA. The instability of Me3TFB–Me4PA in water is surprising, becausethe high number of hydrophobic methyl groups was anticipated to preventwater from a nucleophilic attack.To study the COF stabilityin water, 1 M NaOH, 1 M hydrochloricacid, dichloromethane (DCM), and 3TFB–Me4PA and TFB–Me4PA started to decompose at 363 and 412 °C, respectively. This is comparable to thethermal stability of Me3TFB–PA and Me3TFB–BD.16 It is anticipated thatthe stability of the other COFs in this series will be similar.The thermal stability wasdetermined by thermogravimetric analysis(TGA); Me13C cross-polarizationmagic angle spinning ssNMR (13C CPMAS ssNMR) spectra wererecorded to investigate the structure of the COFs. We used two differentspinning frequencies to distinguish COF signals from spinning sidebands . Compared to the spectra of the starting materials,the 13C CPMAS NMR spectra of COFs show a strongly attenuatedsignal around 190–200 ppm, characteristic for aldehyde carbons,and a signal around 160 ppm, which can be assigned to the imine carbons, before double-checking whether the signals appear at approximatelythe same chemical shifts in ssNMR. At first, we applied our CP contacttime-dependent analysis to assign TFB–PA with the help of Me3TFB–PA and TFB–Me4PA . As the methylated COFs are derivativesof the unmethylated COFs, and a methyl substituent typically has onlya small effect on arylic chemical shifts, we expect comparable chemicalshifts, enabling one to assign all NMR signals by comparing the spectraand CP build-up curves (3TFB–PA and TFB–PA/TFB–Me4PA)that the assignment matches the one measured with 2D HETCOR NMR.37 We now implemented this method in its full potentialto our extended series, including TFB–BD and its methyl derivatives.TFB–BD has also been assigned previously, though no experimentalsupport was shown.29 Hence, we measuredthe CP build-up for TFB–BD . All samplesare microporous, which means that the original p/p0 range from 0.05 to 0.3 is not applicable.Therefore, lower relative pressures have been used for the linearfit, and the Rouquerol criteria39 wereapplied. The same pressure range has been used for the linear fitof the same COF. The BET surface areas for all COFs and both activationmethods are displayed in 2/g. For TFB–PA,36 TFB–Me2PA,26 TFB–BD,40 and TFB–Me2BD,28SBET has been reported before, and the highest reported values are 1571,144, 1948, and 821 m2/g for these COFs, respectively. ForTFB–Me4PA, which is reported in a patent, we couldnot find any SBET value. However, TFB–Me2PA was measured to have a SBET of 680 ± 55 m2/g and TFB–Me2BDof 1026 ± 168 m2/g, which are, to the best of ourknowledge, the highest reported values so far for these specific COFs.For TFB–BD, the BET surface area was found to be 1514 ±97 m2/g. The reported value in the literature is approximately29% higher, which is likely due to the supercritical CO2 drying. This is a mild activation method that prevents pore collapse.The exact values of all BET surface areas can be found in Figures S43–S50.Nitrogen sorption experimentshave been carried out to determine the porosity and BET surface areaof all COFs. All samples were measured in triplicate to check therepeatability. The isotherms can be found in 3TFB–PA andMe3TFB–BD.164PA/Me3TFB–Me4PA, for which the BET surface areas are approximately the same. Itmust be noted that the synthesis of TFB–Me4PA ischallenging and shows a low repeatability. The increase in SBET for aldehyde-methylated COFs is in linewith the crystallinity of the COFs. The magnitude of this increasevaries for the different COFs. The expansion of this previous resultsimplies an overall trend of increasing BET surface area by employinga methylated aldehyde node. It is, however, important which buildingblock is methylated, because SBET decreaseswhen the amine linker is methylated. In 3TFB–PA,TFB–BD, and Me3TFB–BD), in which we addedincreasingly more methyl groups. These findings are in line with ahigher C=O aldehyde to C=N imine stretch ratio in FT-IRspectra for non-methylated COFs compared to their methylated equivalents.This leads us to the conclusion that the position of the methyl groupis crucial for the porosity of the COFs. We hypothesize that the stericdemand of the methyl group plays a role in the COF formation and thatthis affects the COF formation in two ways. As reported previously,16 the methyl groups reduce the conjugation ofthe carbonyl groups with the π-system of the aromatic Me3TFB ring, leading to a smaller rotational barrier after methylation.The smaller barrier enhances the reaction rate of the conformationalchange by several magnitudes, and the preferred orientation of thealdehyde node for the framework formation is achieved faster, whichresults in a more regular framework. On the other hand, when the stericdemand is present on the amine linker, the nucleophilic attack ofthe amine group at the carbonyl group can be already more difficultand is less likely to happen, which leads to the observed decreasein the BET surface area. This line of thought fits with the restrictedsynthesis based on steric hindrance as discussed based on the FT-IRand PXRD patterns.We have recently reported that the BET surface area increases dueto methylation of the aldehyde for MeNot only the structure but also the activationmethod plays a crucialrole in the porosity of COFs. To investigate the stability of theCOFs in this study, all COFs were activated at 120 °C in thetwo different ways that we mentioned before: vacuum versus ambientpressure. Afterward, the BET surface area was measured for all samples,and to correct for the difference in BET surface area due to differentpore sizes, the retained BET surface area has been calculated 1.SBET, meaning that the frameworks are less proneto pore collapse. The magnitude of this effect varies from COF toCOF, being the smallest for TFB–Me4BD/Me3TFB–Me4BD with only 5% difference and the largestwith 70% difference for TFB–PA/Me3TFB–PA.For most methylated COFs also the standard deviations are lower thanthe ones for the non-methylated aldehyde COFs with the exception ofTFB–PA/Me3TFB–PA and TFB–BD/Me3TFB–BD which have roughly the same standard deviations.Based on these findings, Me3TFB can reduce pore collapseand lead to robust COFs.Again, the methylatedaldehyde nodes lead to an increase in retained The UV absorbanceof the COFs was determinedwith diffuse reflectance spectroscopy. In Figure S51).Hence, we used Origin to calculatethe first derivative and usedthe inflection point for additional information. Data analysis withthis approach shows that for COFs with a methylated amine linker,the absorbance spectra red-shifts, whereas a blue shift is observedfor methylated aldehyde nodes 4A–C. Met3TFB–PA to Me3TFB–Me4PA, the absorption edge does not increase, but for the otherthree COF series, this trend still applies. These findings furtherstrengthen our mathematical approach to analyze diffuse reflectancespectra by comparing their inflection points.Traditional analysis of theabsorption edge leads to the same findingswith one exception: the trend in which the spectra red shifts basedon the number of methyl groups does then not apply anymore. Additionally,from MeFigure S52). All the trends we found lead tothe possibility of pre-designing the UV absorbance based on the choiceof building blocks.All changes inabsorbance are also visible by eye, because theisolated COF powders have different shades of yellow shows that the Me3TFB–COF structures are bended,while the TFB–COF structures are flat. Such a difference wasnot found upon the methylation of the amine linker. We hypothesizethat the observed bending from the methylated aldehyde node decreasesthe delocalization of the entire conjugated system, which would resultin an increased excitation energy, leading to a blue shift.The bathochromic shift upon methylationof the amine linker canbe explained by the inductive, electron-donating effect of methylgroups. The hypsochromic shift for methylated aldehydes can, however,not be explained by this effect. The DFT structure optimization ofthe COF sheets . The optical band gaps for TFB–COFs are in the range of2.72–2.52 eV, and methylated aldehyde nodes result in slightlyhigher band gaps of 2.94–2.56 eV. This result matches withthe observed blue shift upon methylation of the aldehyde node. Upontetramethylation of the amine linker, the optical band gap decreases.The energies of the band gaps as well as the observed trends showa better match with the analysis of the UV spectra via their inflectionpoints compared to the traditional analysis via the absorption edge.This supports again that diffuse reflectance spectra with S-shapedcurves can be analyzed by their inflection point. Overall, the bandgaps do not change considerably, which means that methylation canbe used to fine-tune the UV absorbance toward the desired applicationwhile getting a predictable porosity.Fundamental knowledge on how the UV absorbance and the opticalband gap are influenced by its COF structure is of importance to furtherdevelop photocatalytic applications. To study the effect of methylgroups on the optical band gaps, we converted our absorbance spectrainto the Kubelka–Munk function,3TFB) aldehyde nodes with either methylated or non-methylated p-phenylenediamine or benzidine linkers have been synthesized,and imine COF formation was proved by FT-IR spectroscopy, BET surfacearea analysis, and PXRD measurements. Pawley refinement yielded theunit cell dimensions, which were compared to the computationally modeledstructures. For the first time, the signals of the ssNMR spectra ofTFB–BD, Me3TFB–BD, and Me3TFB–Me4BD could experimentally be assigned using contact time-dependentCPMAS NMR and comparing the different COF derivatives. An in-depthstudy of the COF series revealed several trends in their propertiesbased on the methyl substitution pattern of their corresponding buildingblocks. First, COFs with Me3TFB show larger crystallinityand therefore increased BET surface areas up to approximately 2100m2/g. In addition, the COFs are less prone to pore collapse,which was proven by comparing vacuum activation with drying in a regularoven. Second, it is of importance which building block is methylated, since—in contrast to their aldehyde counterparts—COFswith methylated amine linkers show a decrease in crystallinity andBET surface area. This is likely attributed to the higher steric demandof the methyl groups, which is supported by the third trend: the moremethyl groups are attached to the amine linker, the stronger are thedecreases in crystallinity and BET surface area. In addition to thesteric influence on the conjugation of the carbonyl groups with thearomatic ring, we hypothesize that the methyl groups on the aminelinker sterically hinder the COF formation. An amine methylation doesnot show a significant effect on the robustness of the COF againstvacuum activation, which is again in contrast to their aldehyde counterparts.Therefore, the position of functionalization within the COF buildingblocks is crucial.Multiplecombinations of non-methylated (TFB) and methylated (Me3TFBis used as a building block, which leads to a small increase in theoptical band gap compared to TFB–COFs. We hypothesize thatthis is caused by the decreased conjugation caused by the methyl groups.COFs with methyl groups on the amine linkers show a red shift of theabsorbance and smaller optical band gaps, which is in line with thebathochromic effect expected from methyl groups.Fourth, the absorbance spectra blue-shiftsif MeTo conclude,the effect of methyl substitution patterns on highsurface area COF properties can serve for an enhanced rational designof COFs. Furthermore, it allows one to fine-tune the optical bandgaps, which can be relevant for photocatalytic applications of COFs.


Approximately 500 thousand tons of cocoa bean shells (CSs) are generated annually and treated as waste. However, their composition is of great nutritional, technological, and economic interest due to their dietary fiber (46.4 to 60.6%), protein (11.6 to 18.1%), and lipid contents (2 to 18.5%), as well as the presence of flavonoids and alkaloids. Thus, this study aimed to obtain CS flour by milling the CSs, characterizing the flour according to its chemical composition and functionalities, and then applying it in the production of cookies, substituting a wheat flour portion  with CS flour. Cookies were characterized in terms of water, lipids, proteins, phenolic (PC), and total flavanol (FLA) contents, and specific volume (SV), hardness (H), and L*, a*, and b color scale parameters. Increasing the amount of CS showed positive results, as the cookies were enriched with PC  and FLA  but increased hardness (353 to 472 N). By associating the responses, it was concluded that the wheat flour replacement with 30% CS presented values of PC and FLA 3 and 1.6 times higher than the control and could be a formulation of interest to consumers. Cocoa bean shells (CSs) are a byproduct of the cocoa industry. They are removed together with the germ before or after roasting  and treaCocoa derivatives are widely used in the food industry, being applied as ingredients in many products as they provide flavor characteristics that are highly appreciated by consumers . In addiThe appreciation of the utility of agricultural wastes is increasing due to the scarcity of natural resources. Thus, there is great interest in applying these materials as food ingredients with added value. Some works that value CSs have been reported ,4,7. TheRegarding the application of CSs in food products, recent studies have been directed toward applying this material for nutritional enrichment. Jozinovic et al.  producedCookies are frequently consumed because they are considered practical and inexpensive food. They are bakery products poor in some essential nutrients, as they are made from wheat flour, sugar, and fat. To increase their nutritional content and functional appeal, one alternative is to add other ingredients.Handojo et al. , Barros Therefore, this study produced cookies by partially substituting wheat flour with CS flour, aiming to obtain products enriched in bioactive compounds. The CS flour was extensively characterized before its inclusion in cookies. The cookies were chemically and physically characterized, and an objective function was used to associate the desirable and undesirable attributes, suggesting a percentage of replacement that optimizes the incorporation of CSs in cookies.The application of CSs flour directly in the food matrix is a possible approach to enriching products with bioactive compounds in a simplified and more economical way when compared to the methods that provide the recovery of these compounds from the solid matrix for later application. Furthermore, from this application, one can benefit from other characteristics of CS flour, such as its composition rich in fibers and proteins. For this reason, it is essential to evaluate the composition and functionalities of the flour, as well as its effects on product characteristics.CSs were donated by a cocoa industry . Margarine , brown sugar , refined sugar , wheat flour , chemical leavener , and soybean oil  were all purchased in the local market. The reagents used were n-hexane from Merck ; sodium hydroxide, acetone, glacial acetic acid, anhydrous sodium carbonate, and hydrochloric acid from Synth ; sodium dodecyl sulfate, Folin–Ciocalteau reagent, 4-(dimethylamine) cinnamaldehyde (DMAC), and standards of gallic acid and (-)-epicatechin from Sigma-Aldrich . All reagents had purities ≥ 98.0%.https://www.marconi.com.br/produto/245/mill-with-vertical-rotor-fixed-hammers- (accessed on 19 December 2022)) to obtain CS flour. This flour was characterized by particle size distribution using sieves ), allowing the calculation of the average particle diameter according to ASAE standards [The CSs, as received, were ground in a mill ), lipid content with a high-temperature solvent extraction system Ankom XT10 Extractor ) ). To convert the total nitrogen content result into protein content, the obtained value was multiplied by a factor of 6.25 [The CS flour was then characterized in terms of moisture  using a of 6.25 . The CS  of 6.25  was follv/v/v) according to the methodology suggested by Hammerstone et al. [To quantify the total phenolic compound (PC) and total flavanol (FLA) contents, CSs and cookie extracts were obtained by contacting 0.5 g of a solid sample, previously cold defatted with hexane, and 5 mL of an acetone/water/glacial acetic acid solution  from two experimental determinations [w/v), and 2.5 mL of 10% Folin & Ciocalteu solution (v/v). After mixing, the samples were kept in a dark environment for 2 h. The determinations were performed in a spectrophotometer ), reading the absorbance at 760 nm.The methodology of Folin & Ciocalteu  was usedinations . For thiFor total flavanol content determination, the 4-(dimethylamino) cinnamaldehyde (DMAC) methodology was utilized ,28, usinThe CS and wheat flours were evaluated according to their nitrogen solubility indices (NSI) following Morr et al.  with modw/v) [https://speciation.net/Database/Instruments/Shimadzu-Europe/UV1650PC-;i1296 (accessed on 19 December 2022)). Equation (1) presents the calculation used to determine the stability index of the emulsion (ESI).Emulsifying activity (EAI) and stability (ESI) indices were also determined, as described by Pearce and Kinsella . Emulsiow/v) . AbsorbaThe CS and wheat flours were also evaluated according to water (WAC) and oil (OAC) absorption capacities . The solCookies were formulated as suggested by Pareyt et al.  with modhttps://www.kitchenaid.com/countertop-appliances/stand-mixers/tilt-head-stand-mixers/p.value-bundle-artisan-series-5-quart-tilt-head-stand-mixer-with-flex-edge-beater.ksm150feer.html? (accessed on 19 December 2022)). Then, water was added for two minutes of further homogenization. After adding the flours and yeast, the mixture was homogenized for 2.5 min. The dough was portioned into 16 ± 1 g units, and each portion was molded into a circular shape approximately 6 mm thick and 5 cm in diameter. The samples were baked for 7 min at 165 °C in a Programmable Turbo Oven with convection ).For preparation, first, a mixer was used to homogenize the margarine and sugars for three minutes ) to calculate the specific volume [All cookie formulations were evaluated after baking in terms of mass and volume .https://www.stablemicrosystems.com/TAXTplus.html (accessed on 19 December 2022)). A 25 mm diameter cylindrical probe with a 5 kg load cell was utilized per Bárcenas and Rosell’s methods [https://www.hunterlab.com/en/products/benchtop-spectrophotometers/aeros/ (accessed on 19 December 2022)) according to the characterization performed by Castro et al. [Cookies were analyzed in terms of hardness by using a texturometer  was proposed as the ratio between the responses that must be maximized to the responses that must be minimized (Equation (2)) [PC and FLA and the specific volume are considered the variables to be maximized. Conversely, the coloration parameter L* and the hardness are variables to be minimized.A joint analysis among results from chemical characterization (phenolic and flavanol contents) and instrumental methods (coloration and hardness) was performed as a first effort to delimitate the percentage of replacement of wheat flour by CS flour. An optimized objective function (ion (2)) . The conPC is the content of total phenolic compounds, FLA is the content of total flavanol compounds, SV is the specific volume, L* is the color parameter, and H is the hardness. These variables were proportionally coded from 1 to 10, with 1 being the lowest value and 10 being the highest value observed for each variable. This coding was performed so that all variables influenced the OF by the same order of magnitude.Here, ® program ).The results from the characterizations are expressed as the means of at least two determinations. These results were compared by analysis of variance using the Duncan test  at a conThe particle size distribution of the CS flour used in the cookies production is shown in One of the concerns that there has been with the application of CSs in food products is the possible contamination of the material with mycotoxins or heavy metals . Cocoa bBased on the results presented in Regarding CS contamination with heavy metals, studies have reported that the metals most frequently found in cocoa products are nickel (Ni), cadmium (Cd), chromium (Cr), and lead (Pb). The factors that favor contamination by heavy metals may be using fertilizers, pesticides, and insecticides and contacting metal equipment during fermentation and drying . CSs mayIn Brazil, ANVISA published a Normative Instruction in March 2021, updating the maximum tolerated limits of contaminants relevant to consumers’ health. The cadmium and lead contents allowed in chocolates and cocoa-based products are 0.3 and 0.4 mg/kg, respectively, for products with more than 40% cocoa. For products with less than 40% cocoa , the levOne of the factors of interest in the use of CS is its composition containing bioactive compounds. The main bioactive compounds in CS are flavanols  and alkaloids (theobromine and caffeine) ,10,55. T2 > 0.99963, was obtained. LOD and LOQ values were 1.12 µg of gallic acid equivalents (GAE)/g of sample and 3.75 µg of gallic acid equivalents (GAE)/g of sample.Regarding the Folin & Ciocalteu  method, 2 > 0.9991) was obtained. The following values were calculated for LOD and LOQ, 0.27 µg of epicatechin equivalents (EE)/g of sample and 0.90 µg of epicatechin equivalents (EE)/g of sample, respectively. The determined LOQ values were lower than the lowest point of calibration curves.For total flavanol content determination using the DMAC method with epicatechin as the standard, an average calibration curve  and 1.38 ± 0.05 (mg EE/g sample), respectively . These vImportant to mention that the methodologies of Folin–Ciocalteu and 4-dimethylaminocinnamaldehyde (DMAC) used to quantify phenolic compounds and flavanols, respectively, were previously evaluated and validated by Ma et al. . Reprodup > 0.05). These values are similar to those reported by Soares et al. [The solubilities of the proteins present in the CSs were similar in water and saline solution, with values of 38 ± 1 and 40 ± 5%, respectively , respectively, are similar to those reported by Soares et al. , and Del2/g with an ESI of 81 ± 1%, which indicates that after 10 min, 81% of the emulsion persisted. The EAI value found here is lower than that reported by Soares et al. [2/g) and similar to the lowest values reported by Cvitanovit et al. [2/g. It is worth mentioning that these results are very dependent on the batch of material used, which depends on the great variability in composition associated with agro-industrial residues, as discussed for the observed variation in the CS lipid content.The CS flour emulsifying capacity was evaluated by determining the EAI, and its stability was evaluated by determining the ESI. CS flour had an EAI of 60 ± 1 mt et al.  at approm/m). The dough became heterogeneous and brittle with more CS flour, making it challenging to form cookies. Cookies were formulated by replacing 0, 10, 20, 30, and 40% of the total mass of wheat flour with CS flour. Cookies could be formulated with up to 40% CS flour (p > 0.05). Chagas et al. [Cookies were characterized by their composition in terms of moisture, lipids, proteins, PC, and FLA . Consides et al.  evaluates et al.  also evas et al. ,12.p < 0.05, In terms of the lipid content, it was noted that there is a significant increase from the C0 formulation to the C40 formulation , was calculated. Graphically, the 0° angle is considered red, the 90° angle is yellow, the 180° angle is green, and the 270° angle is blue . °Hue vap > 0.05). Chagas et al. [The substitution of wheat flour for CS flour did not influence the spreading factor ) was proposed as a first effort to delimitate the percentage of replacement of wheat flour by CS flour. It can be seen that from formulation C30 onward, undesirable variables  begin to exert a more significant influence on the sample, causing a drop in OF. Therefore, it is inferred that the C30 formulation presents the best balance between desirable and undesirable variables. The C30 formulation presents a phenolic compound content of 2.1 ± 0.1 mg GAE/g sample and flavanol content of 0.170 ± 0.005 mg EE/g sample, values 3 and 1.6 times higher than the control (C0 formulation).This analysis taking into account instrumental texture results, is a first fetch, not intending to substitute the sensory analysis. According to Guiné , instrumThe evaluation of the CS flour regarding its chemical composition and functionalities showed that this material has the potential for application in systems that benefit from these properties, such as bakery products, due to its high water absorption capacity. Increasing the concentration of CS flour applied in the formulations made the cookies darker, indicating a “chocolate flavor” product, and enriched the formulations with flavanol and phenolic compounds. However, a limit for this substitution was observed due to the reduction of the gluten matrix, which plays a fundamental role in bakery products. The main factors that limited the substitution of wheat flour for CS flour to 40% were the loss of the ability to form a homogeneous dough and the increase in the hardness of the cookies.By associating the desirable and undesirable effects of wheat flour replacement with CS flour through an objective function, it was concluded that the formulation with 30% CS substitution showed the best balance among the desirable (enrichment with phenolics and flavanols) and undesirable (hardness) effects, and could be a formulation of interest to consumers. C30 formulation presents 14.2% of CS flour, with values of total phenolic and flavanol contents 3 and 1.6 times higher than the control formulation.


Lentil flour is an important source of minerals, including iron, so its use in food fortification programs is becoming increasingly important. In this study, the potential of near infrared technology to discriminate the presence of lentil flour in fortified wheat flours and the quantification of their mineral composition is evaluated. Three varieties of lentils  were used to produce flours, and a total of 153 samples of wheat flours fortified with them have been analyzed. The results show that it is possible to discriminate fortified flours with 100% efficiency according to their lentil flour content and to discriminate them according to the variety of lentil flour used. Regarding their mineral composition, the models developed have shown that it is possible to predict the Ca, Mg, Fe, K and P content in fortified flours using near infrared spectroscopy. Moreover, these models can be applied to unknown samples with results comparable to ICP-MS determination of these minerals. Millions of tons of flour from different sources are consumed by people around the world . In thisFlours provide different micronutrients in the diet  through the process of fortification in which nutrients or non-nutrient bioactive components are added to edible products. Owing to their low cost, high mineral content fortified flours have been used in many public health strategies in underdeveloped countries to combat malnutrition and diseases caused by vitamin or mineral deficiencies ,10. HowePreparations containing flour mixes intended for the processing industry and even for direct sale to the consumer have increased their presence in recent years. However, these products are among the foods with the highest incidence of adulteration ,18,19. BIn this context, several techniques have been evaluated, including authentication based on scanning electron microscopy (SEM) , moleculNIR technology has also been applied to the detection of wheat flour fraud ,51. ThusThe aim of this study is to evaluate the viability of NIR technology in the analysis of wheat flour fortified with lentil flour in order to assess that wheat flours fortified with lentil flour fulfil the declared composition. To this purpose, the study aims to predict the percentage of lentil flour and the final mineral content  of the fortified flours from the NIR recording obtained from these flours.−4 Joules, belonging to the guaranteed brand “Harina Tradicional Zamorana”. Subsequently, 60 lentil flours were manufactured. Lentil flours were obtained by grinding whole lentils in a micro-hammer mill  and sieving them to a final particle size of less than 0.140 mm. Three different varieties of brown lentils grown in Spain  were used to obtain the flours. The lentils were supplied by the Centro de Legumbres  and harvested from 10 different plots in the regions of Zamora and Salamanca (Spain) in order to achieve as much variability in their mineral content as possible. Finally, 90 fortified flours were prepared by mixing wheat flour with different amounts of lentil flour. In all cases, 50 g of flour were prepared. The wheat flours and lentil flours used in the mixture were chosen randomly, taking care that for each level of fortification 10 mixtures were prepared with each lentil variety. The fortified flours were prepared by mixing wheat flour with 25, 50 and 75% of the different lentil flours.A total of 153 flour samples were analyzed as described in 3 and a Milestone digestion microwave system was used. Concentrations were determined using an Agilent 7800 ICP mass spectrometer  with the following operating conditions: Reference Power 1550 W, Plasma Air flow 15 L/min, Auxiliary Air flow 0.9 L/min and Nebuliser Air flow 0.99 L/min). The quantification was carried out by using certified standard solutions , and the results were expressed in mg/kg of flour.Mineral concentrations were determined by ICP-MS . Samples (0.2 g) were introduced in Teflon vessels with HNOThe flours were analyzed with a Foss NIR System 5000 monochromator . A 1.5 m 210/210 remote reflectance fiber probe (Ref. No. R6539-A) was used with a 5 × 5 quartz window area. The spectral range used was between 1100–2000 nm at 2 nm intervals. The measurement was carried out by direct application of the fiber-optic probe to 10 g of flour, as shown in Firstly, a principal component analysis (PCA) was performed to detect spectral outliers using the Mahalanobis distance as a criterion for the validity of the samples , consideCalibration was then carried out using the spectra of the samples and their respective mineral content data analyzed in the laboratory as well as the percentage of lentil flour present in the fortified samples. Four different spectral treatments were applied to correct the possible scattering effects of the samples: multiplicative scatter correction (MSC), standard normal variant (SNV), DeTrend (DT) and SNV-DT. In addition, four different derivative math treatments were used. The calibration equations were generated using the modified partial least squares (MPLS) regression method. The equation generated was developed by cross-validation with the selection of an optimal number of factors to avoid over-fitting. To evaluate the developed models, the performance deviation (RPD) and the cross-validation mean square error (SECV) were analyzed.t-test for paired values.The selection of the best calibration equation was made based on the coefficient of determination R^2 and the RMSEC . Finally, the equations obtained were applied to the validation set which was not used for calibration (33 samples). The values predicted from the equations generated were compared with the mineral content data determined in the laboratory using the Student’s Discrimination of the flours according to the percentage of lentil flour was carried out using the Residual Mean Squares (RMS-X) supervised pattern recognition method. The same spectral pre-treatments as described above were tested in this case. Some 80% of the samples were used to generate the model, and the remaining 20% were used for subsequent validation. The discrimination models obtained were evaluated according to the percentage of well-classified samples during both calibration and external validation. The sensitivity of the models was calculated according to the expression proposed by .The raw spectra of the flours show a large vertical shift with respect to the baseline, which affects the whole spectral region analyzed. These differences could be related to the scattering behavior of samples with different particle sizes. Scattering is the main source of spectral variability when analyzing milled products such as flour although the use of pre-treatment for the original spectra allows for the elimination of particle size effects ,64. ConsThe Norris–Williams spectral derivative was used where the first digit was the number of the derivative, the second was the interval during which the derivative was calculated, the third was the number of data points in an average or smoothing, and the fourth was the second smoothing . For mosIn an initial stage, the ability of discriminant models to classify samples attending to the different percentages of the lentil flour contained in the mixtures was evaluated. For this purpose, the Residual Mean Squares method was used. Lens culinaris subsp. Macrosperma; however, the Pardina variety belongs to subsp. microsperma [Furthermore, the possibility of discriminating the fortified samples according to the variety of lentil used in the production of the flour was tested . Althougrosperma , which cNIR technology has already been applied in the discrimination of flours with correct classification values above 90% in sweet potato flour and its adulterations  and in tp < 0.05) in the flours of the Castellana variety. The magnesium contents did not show statistically significant differences between the different varieties (p ≤ 0.05). However, the Castellana variety shows a statistically (p ≤ 0.05) higher calcium content than the Guareña and Pardina varieties. The concentrations found for all minerals in the present study agree with those described for green, yellow and red lentil flours [l flours ,72.p < 0.05) between the three lentil varieties owing to the large variability found between the different growing areas from which the lentils were collected. As reported, the micronutrient content of lentils appears to vary greatly according to their location [Lentils have traditionally been considered a good source of iron ,73. Theilocation , the envlocation  and the location .p < 0.05) in lentil flours than in wheat flours.The results obtained show that the concentrations of all the minerals analyzed are significantly higher  was first performed. The spectral variability explained was above 99% in all cases, and between 6 and 10 principal components were required for each mineral analyzed. Calibrations were performed by modified partial least squares (MPLS) regression for each of the minerals after removing those samples that did not satisfy the spectral criteria of Mahalanobis distance (H distance). Spectra with H values equal to or greater than three were therefore discarded.Some 120 samples were used to develop the calibration models. The best performing mathematical treatment and the calibration parameters for all minerals analyzed  are shown in The calibration equations developed take into account all the wavelengths recorded when obtaining the NIR spectrum. However, not all of them contribute to the equation in the same way. Each wavelength is multiplied by a coefficient (β coefficient), the higher the value of β, the greater the contribution of that wavelength to the predictive ability of the equation developed. The results obtained in this study show that the lengths between 1212 and 1282 nm have high β coefficients in the equations of all the minerals analyzed. High coefficients are also observed at 1578 nm for Ca and Fe. In the case of K, high β coefficients are observed between 1114 and 1184 nm.From the equations obtained, an internal validation was carried out through a cross-validation of the samples used in the development of the equations. To evaluate the performance of the models, the RSQ (R squared), the RPD (Relative Percent Deviation) and the SEP (standard error of prediction) values were used. The RSQ coefficient measures the correlation between the content in each mineral predicted by the model and the value obtained in the laboratory. The RPD value is the ratio of the yield to the deviation. The SEP provides an indication of the precision of our results. In the internal validation process, samples with high residual values were eliminated using the criterion T > 2.5.The results of the statistical descriptors of the internal validation and the correlations between the mineral content measured in the laboratory and that predicted by NIR are shown in RSQ values are highly satisfactory for P, Mg and K. For Fe, a lower value is observed, as was previously shown when developing the calibration equation. Poor calibrations for Fe have been previously reported for legumes ,83. For Taking these values into account, the models developed can be considered good for all the minerals analyzed and all the equations developed fulfilling this criterion.t-statistic and the RMSE error. t-test for paired values when comparing the reference data (obtained in the laboratory by ICP-MS) and the values obtained from the NIRS equations for these 33 samples. The p-values were higher than 0.05 for all the minerals evaluated, which means that there are no significant differences between the values provided by the two methods. The RMSE (root mean standard error) obtained ranged from 0.006 for Mg to 8.590 for Fe. The high RMSE value obtained in the prediction of Fe indicates a lower accuracy in the determination of Fe content. In spite of this, the results obtained show that the prediction equations were satisfactory for determining the mineral content of lentil flours and flours fortified with lentil flour.In order to check the validity of the predictive models developed, an external validation was carried out. For this purpose, 33 samples  which had not participated in the development of the models were analyzed. For this purpose, the calibration equations obtained were applied to the spectra of the samples reserved for external validation. The suitability of the model developed was assessed by comparing both results through the Student NIR technology has proven to be an effective tool for establishing the presence of lentil flour in fortified samples and for the quantification of its mineral content. Flours could be satisfactorily discriminated according to the percentage of lentil present in them. Furthermore, promising results were obtained in the discrimination of the lentil variety used. Regarding the mineral composition, the models developed have proved to be suitable for the determination of the Ca, Mg, Fe, P and K content of the flours. The range of applicability of the calibration equations obtained by NIR with a fiber optic probe is comparable to that obtained by the reference technique (ICP_MS). The multiparametric character of NIR technology combined with its low cost and rapid analysis make it a very promising tool in the field of vegetable flours and their formulations.


Stress-strain constitutive relations and Poisson’s ratios are fundamental properties of naturally occurring materials, based on which their mechanical applications can be designed. The lack of tailorability and restricted margin for such critical properties severely limit the bounds of achievable multi-functional engineering designs using conventional materials. Through analytical and numerical analyses, supported by elementary-level physical experiments, we have proposed a kirigami-inspired hybrid metamaterial with programmable deformation-dependent stiffness and mixed-mode multidirectional auxeticity. The metamaterial can transition from a phase of low stiffness to a contact induced phase that brings forth an extensive rise in stiffness. Uniform and graded configurations of multi-layer tessellated material are developed to modulate the constitutive law of the metastructure with augmented programmability in two- and three-dimensional spaces. The proposed metamaterial will lead to extreme lightweight functional designs for impact resistance, shape morphing, multidirectional deformation, vibration and wave propagation control, where the capabilities of intrinsic material can be most optimally exploited.  •Kirigami-inspired hybrid metamaterial with contact-induced programmable features•Deformation-dependent stiffness and mixed-mode multidirectional auxeticity•Uniform and graded configurations of multi-layer tessellated materials•Extreme lightweight functional designs for most optimal material utilization Mechanics of materials; Mechanical property; Metamaterials Kirigami, otherwise known as “paper-cutting”, is similar to the better known art of Origami, which involves only folding paper. Kirigami allows for both folding and cutting of a thin sheet to create intricate three-dimensional designs. Kirigami and origami can be seen as models to fabricate deployable structures of a wide spectrum of sizes that may transform their shape without any complicated joints or sophisticated electro-mechanical means . A vitalWith significant potential for a wide range of engineering applications, mechanical metamaterials form a domain of cutting-edge research in the present age . One of Recently auxetic metamaterials with negative Poisson’s ratios , which cFor a given origami based metastructural pattern, variation of stiffness with the change in geometric dimensions is well-explored. It has also been shown that a sharp increase in stiffness can be achieved by distorting some origami units (such as forced inversion of vertices) . HoweverKirigami and Origami-inspired metamaterials are receiving increased attention from the scientific community in recent years because of their exciting properties such as space-filling periodicity , shape sMotivated by the objectives of programmable modulation of force-deformation constitutive laws and multi-modal Poisson’s ratios, unit-cell refer to B of the Firstly, the analytical formulation of the rigid-origami kinematic motion of the proposed unit-cell would be validated through idealized structural simulations. Subsequently, the effects of microstructural geometric parameters on in-plane Poisson’s ratio would be analyzed theoretically. Further, the effects of microstructural geometry would be studied on the constitutive laws of the metamaterial during compressive and tensile loading. Later in this article, the proposed 2D tessellation will be extended to develop a 3D stacked kirigami-based metamaterial with multidirectional auxeticity, which can potentially find critical applications in sensors , adaptivIn this section, first we present detailed geometry of the proposed kirigami metamaterial describing the underlying parameters for modulating mechanical properties of the structural system. Unit-cell of the proposed kirigami metastructure is obtained by taking a uniform truncated octahedron of side length shown in A as a teAn ‘asymmetric’ unit-cell shown in C is obtarefer to E, leadinrefer to .We study the rigid origami motion of the unit-cell shown in B by rigirefer to . and II. Tangent Poisson’s ratio (obtained by taking derivative at each instance of the deformation path). One may note that the definition for secant Poisson’s ratio and tangent Poisson’s ratio stated here are alike Poisson’s ratio and Poisson’s function defined in  1 and 2 . Based oIn By definition, a single unit cell can define the entire periodic metastructure when appropriate boundary conditions are implemented. For example, the Poisson’s ratios of a hexagonal honeycomb lattice can be evaluated by considering only one unit cell as per well-established literature . A similTo study the effects of variation in microstructural geometry on the in-plane (YZ plane) Poisson’s ratio Of interest, whenever a variation in connector length To establish a deformation and microstructure-dependent stiffness that can predictively be modeled as a function of the applied far-field deformation, the constitutive relation of the meta-sheet made of symmetric unit-cells under longitudinal load is shown in Consider the meta-sheet as shown in Consider the meta-sheet under tensile loading as shown in refer to .It becomes clear from the discussions of the preceding paragraphs that the steep rise in stiffness of the metamaterial can be programmed as a function of deformation (or applied end-force of tensile and compressive mode) according to the explicit requirement of a system or structure. It can further be noted that the proposed metamaterial is capable of exhibiting non-invariant force-deformation constitutive behavior under tensile and compressive mode of loading, which is contrary to the common mechanical behavior of conventional naturally occurring materials. In this context, it may be noted that such programmable large deformation constitutive modeling is normally not possible in lattice metamaterials.A graded microstructure is often implemented in metamaterials to achieve variable stiffness in the system or structure as to where it is needed . Through the proposed metamaterial, we demonstrate that it is also possible to accomplish variable stiffness as to when in the deformation process it is desired . Thus, we can now propose meta-sheets developed with unit-cells of different geometrical attributes to vary stiffness as to where in the structure and as to when in the deformation process it is needed. One can observe that to make a meta-sheet as proposed in In the case of compressive loading for symmetric unit-cells A–3D, botWe have proposed two kinds of graded meta-sheets based on the direction of gradation. These meta-sheets can be designed with any numerical configuration of unit cells. However, for this study, we have considered configurations based on the ease of graphical representation and to portray the interesting trends. In Earlier, we have shown that in compressive loading, edges of the extrusions of vertically adjacent unit-cells make contact. For proper edge-to-edge contact between extrusions pointing toward each other , their lengths The stiffness curve can be divided into three broad sections defined by the first and final contacts in the meta-sheet. Using Each row of Configuration-1 consists of pairs of the 13 unit cells arranged as described earlier, and each column consists of 26 unit cells of a kind. Because all 26 unit cells cannot be shown in the grids, square cells containing dots are added to represent the intermediate unit cells, columns or rows. In accordance with the color gradient scheme, the penultimate and final symmetric unit cells at the end of each row are represented by a white square cell. A slight decrease in stiffness in the second section is shown by replacing either the final unit-cell or both the final and penultimate unit-cells of odd columns by a symmetric unit-cell in configurations 2 and 3, respectively. By replacing asymmetrical unit-cells with symmetric unit-cells, contribution to the net stiffness in the second section decreases because contact happens only at the very end of the section. As expected, a steep drop in stiffness in the second section is seen when the final unit-cell of every column is replaced by a symmetric unit-cell in Configuration-4. Further decrease in stiffness can be seen with Configuration-5, where the final unit-cell of every column along with the second-last unit-cell of odd columns are replaced by symmetric unit-cells. The ‘asymmetric-I’ unit-cells present at the created interface are replaced by ‘asymmetric-II’ unit-cells in these configurations. One way of increasing the stiffness in the second section is by replacing the two columns of symmetric unit-cells in Configuration-1 with columns of asymmetrical unit-cells with an In One can note that at the interface of two different sets in a column, two different unit-cells would be making contact  in the 2D tessellation (column and row-wise), it is possible to obtain piece-wise programming of stiffness  and force-displacement constitutive behavior with an increased scope of modulation.In this section, we present the strategies to extend the proposed 2D hybrid origami-kirigami tessellation to 3D domain. The particular hinged edges of out-of-plane extrusions can be observed in the orthogonal top view of the 3D tessellation shown in Previously in In the proposed kirigami-based hybrid metamaterial, the displacement or force applied at its two ends can be referred to as ‘distant actuation’. The word ‘distant’ describes the ability of the metamaterial to display a change in its shape (depending on the nature of in-plane and out-of-plane Poisson’s ratios) far away from the point of actuation of either displacement or force. Thus, this external far-field actuation enables local shape modulation of the metamaterial based on its microstructural attributes, which can be varied as per the requirement of an application. For the 3D tessellation, similar programmable force-displacement constitutive behavior can also be obtained as in the case of 2D tessellations.In general, constitutive relations and Poisson’s ratios are fundamental properties of naturally occurring materials, based on which their engineering applications can be designed. The lack of tailorability and restricted margin for such critical properties severely limit the bounds of achievable multi-functional engineering designs using such materials. Over the last decade, the materials community is paying increasing attention to the possibilities of developing artificial material microstructures based on application-specific multi-functional demands, rather than solely relying on the properties of natural materials. There exists a strong rationale to be able to program the constitutive relationships for extreme structural designs, where the capabilities of intrinsic material can be most optimally exploited based on application-specific requirements. In this article, we have shown a range of such unprecedented mechanical properties through kirigami-engineered metamaterials.Because the fundamental deformation mechanics of the proposed kirigami-based metamaterial is scale-independent, it can be directly utilized for application across micro to macro scales. In this context note that the macroscale material properties in metamaterials are derived from the ‘structure’ at a much lower length-scale , leading to the aspect of developing new ‘materials’ at the higher length-scale with unprecedented physical properties. In such conceptualization of metamaterials, an adequate ‘relative’ difference between the lower and higher length-scales allows us to define the effective physical properties of a ‘material’  based on the ‘structure’ therein. Further, the force-displacement constitutive behavior, as presented in this article, can, in principle, be converted to the stress-strain behavior by considering the area of applied force and calculating the corresponding strain. It may be noted here that the proposed unit cell (and the 2D and 3D lattices) under the deformation mode explored in this article are 1° of freedom structures if only kinematic motion is considered before any contact refer to . This isAs shown in The vastly possible configurations that can be formed using unit-cells with different geometrical attributes expand the capability of these classes of metamaterials dramatically. In Through analytical and numerical analyses, supported by elementary-level physical experiments, here we have presented a novel kirigami-inspired hybrid metamaterial with programmable deformation-dependent stiffness and mixed-mode multidirectional auxeticity. In the proposed metamaterial, first symmetric and asymmetric unit cells  are developed, the behavior of which is dominated by principles of origami. In the next stage, these modular units are tessellated in kirigami-inspired 2D and 3D domains by designing compatible edge connections. The stiffness and auxeticity of the proposed metamaterials can be modulated as a function of the applied far-field deformation with the microstructural geometric configurations and attributes. A transition from kinematic motion to structural deformation can be programmed to achieve an extreme rise in stiffness in both compressive loading and tensile-loading scenarios as to when in the deformation process it is needed. Through spatially varying graded microstructure with asymmetric geometries, we have further shown that it is possible to modulate piece-wise constitutive curves as per wide-ranging functional demands. This will contribute to advancements in the field of lightweight multi-functional structural systems where the stiffness can be programmed for the most optimum exploitation of the material. It may be noted in this context, unlike conventional materials, the proposed metamaterial exhibits a programmable non-invariant force-displacement constitutive behavior under compressive and tensile modes. The degree of non-invariance can be designed based on microstructural geometry on the basis of application-specific functional demands.Based on the microstructural attributes, the proposed metamaterial can achieve either pure (only positive or negative) or mixed-mode  in-plane and out-of-plane Poisson’s ratios. In addition, the transition between positive and negative out-of-plane Poisson’s ratios during the deformation process can be programmed as a function of the microstructural attributes of the metamaterial. Based on the lateral strain components under applied a far-field displacement , large-scale modulations in shape can be obtained in the developed metamaterial, leading to a distant actuation feature without requiring intricate networks of complex locally implanted sensors, actuators, and controllers. In general, the capability of Poisson’s ratio programming in the 2D and 3D spaces, with the deformation-dependent constitutive behavior modeling , will lead to the modulation of a range of global structure-level mechanical behavior such as shape morphing, direction-dependent static deformation, vibration, wave propagation, and impact resistance. Such exciting features make the proposed kirigami-based hybrid metamaterial a multi-functionally exploitable crucial addition to the class of innovative programmable metamaterials that can find applications in a wide range of mechanical and aerospace systems, transformable architecture, soft robotics, biomedical equipment and energy absorption structures under multi-directional impact loading.tanmoy@iitk.ac.in).Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Prof. Tanmoy Mukhopadhyay . This practically eliminates bending in panels and allows the creases to fold relatively much easily. This can be confirmed from the plots in The analysis presented in this section is general and it takes a non-rigid origami approach by assigning bending stiffness Note that based on the structural analysis methodology described above, the equivalent force-displacement constitutive behavior of each unit-cell (pre- and post-contact) can be obtained. Subsequently, a spring  based analogy can be adopted to obtain the force-displacement curves of the proposed metamaterial by idealizing it as a network of equivalent springs. In such analyses, we consider a displacement-controlled simulation with the assumption of two parallel edges during any stage of deformation, while the vertical displacement is applied perpendicular to these horizontal  edges.Poisson’s ratio can usually be reported in two ways: I. Tangent Poisson’s ratio  and II. Secant Poisson’s ratio . One may note that the definition for tangent Poisson’s ratio and secant Poisson’s ratio stated here are similar to Poisson’s function and Poisson’s ratio defined in Thus, we can express the horizontal length Through Similarly, the vertical length (V) and the vertical length at no longitudinal deformation The horizontal length for no longitudinal deformation The secant Poisson’s ratio is defined as:Thus the in-plane secant Poisson’s ratio for the proposed unit-cell can be obtained from the above equations as:The tangent Poisson’s ratio is defined as:One can obtain the tangent Poisson’s ratio using Through The horizontal length for no longitudinal deformation The secant Poisson’s ratio is defined as:Thus the out-of-plane secant Poisson’s ratio for the proposed unit-cell can be obtained from the above equations as:One can obtain the tangent Poisson’s ratio for out-of-plane direction using The tangent Poisson’s ratio is defined as:Initially, under far-field actuation, the metamaterial undergoes a purely rigid deformation  until contact among edges occurs. At this point, the facets of the unit-cell must deform to allow any further vertical deformation. Therefore, non-rigid behavior is observed in the post-contact regime. Here we have obtained closed-form expressions to quantify the longitudinal deformation required for initial contact in a unit-cell in terms of its geometric attributes. shown in B is defishown in B has a vSince we aim to achieve a transition from rigid to non-rigid deformation through contact, here we have found expressions that set certain constraints on the geometric attributes ed state  must be We have introduced two types of asymmetric unit-cells known as the 'Asymmetric-I′ and the 'Asymmetric-II′ unit-cells. The Asymmetric-I unit-cell is obtained by having two pairs of diametrically opposite extrusions of length Consider the 'Configuration 1′ described in One may note that for edge-to-edge contact between vertically adjacent unit-cells in compression, the For a detailed explanation, we have presented the formation of an interface between two sets in To enable proper contact between unit-cells at an interface we defined that a set would have Asymmetric-II unit-cells at its extreme ends. A set of 'An 'asymmetric-I′ unit-cell . This is also tantamount to having 1° of freedom in the lattice structures before any contact. As explained in We also must consider ‘side-length’ constraints to keep the overall structure rigid. Each flexible hexagon can change its shape, but its side lengths must remain the same as in the original unit-cell. There are three such side length constraints per hexagon, but one of these sides is shared with either the top or bottom hexagonal panels. Therefore, we have two side length constraints per flexible hexagon, and each such constraint is paired with two other flexible hexagons, leading toSo, in total, we have 1 DOF for the unit-cell. This simple analysis is consistent with the derived result in


Food safety is a key issue in promoting human health and sustaining life. Food analysis is essential to prevent food components or contaminants causing foodborne-related illnesses to consumers. Electrochemical sensors have become a desirable method for food safety analysis due to their simple, accurate and rapid response. The low sensitivity and poor selectivity of electrochemical sensors working in complex food sample matrices can be overcome by coupling them with covalent organic frameworks (COFs). COFs are a kind of novel porous organic polymer formed by light elements, such as C, H, N and B, via covalent bonds. This review focuses on the recent progress in COF-based electrochemical sensors for food safety analysis. Firstly, the synthesis methods of COFs are summarized. Then, a discussion of the strategies is given to improve the electrochemistry performance of COFs. There follows a summary of the recently developed COF-based electrochemical sensors for the determination of food contaminants, including bisphenols, antibiotics, pesticides, heavy metal ions, fungal toxin and bacterium. Finally, the challenges and the future directions in this field are discussed. Food safety is one of the issues of most global concern because safe food is essential for promoting human health and sustaining life ,2. The WIn recent years, analytical techniques such as high-performance liquid chromatography , gas chrAn electrochemical sensor is an electronic instrument that can conduct quantitative detection of analytes through a redox process. The electrochemical sensor is usually composed of three electrodes: working, reference and counter electrodes . These tCovalent organic frameworks (COFs), as a new kind of organic porous polymer, are connected to light elements  via covalent bonds ,23. COFsThis review aims to present the recent advances of COF-based electrochemical sensors which have been reported regarding their application to food safety analysis . The curCOFs have received extensive attention due to their wide application. Thus, it is critically important to develop new synthesis technologies to improve their application range. Recently, COFs have become an attractive material in electrochemical sensing because they can improve the selectivity, sensitivity and speed of electrochemical sensors . NeverthUp to now, researchers have reported many synthesis approaches for COFs; five typical synthesis approaches—solvothermal synthesis, mechanochemical synthesis, solvent-free synthesis, microwave-assisted synthesis and sonochemical synthesis—are summarized  and furt2-carbon- [According to the reports, most of the prepared COFs are synthesized by the solvothermal synthesis method. In this method, monomers and solvents are added together in a Pyrex tube. After a freeze–pump–thaw cycle, the Pyrex tube is sealed and placed in an oven. The reaction often requires several days (2–9 d) under a controlled temperature (80–200 °C). After cooling to room temperature, COF is obtained through filtration, washing and drying. It should be pointed out that the solvent combinations and ratios and the reaction time have a great impact on the crystallinity and porosity of COFs in this method . Since O-carbon-  and poly-carbon- . Althougp-phenylenediamine were placed in a mortar and ground for 5 min at room temperature, and then a COF with a light yellow color was obtained. Subsequently, several COFs such as TP-COP [Mechanochemical synthesis has become an alternative synthesis method for preparing COFs due to its being simple, timesaving, solvent-free and operable at room temperature. Rahul Banerjee et al.  developes TP-COP , TpMA [3s TP-COP  and TpMas TP-COP  were alsThe solvent-free synthesis method has emerged as a viable method to prepare COFs owing to its environmental protection, simple operation, low cost-effectiveness and large-scale preparation . Zhenjie2/g) than that prepared by the conventional solvothermal method. In addition to boronate ester-linked COFs, covalent triazine-based framework [The microwave-assisted synthesis method has received more attention due to its advantages of shorter reaction times, higher yields, environmental protection and lower energy consumption . Andrew ramework , melaminramework , enamineramework , cationiramework  and iminramework  have alsThe sonochemical method is capable of facilitating the homogeneity of COFs and accelerating the crystallization rate due to its use of ultrasonic wave, which can produce strikingly high pressure and local temperature, as a heat source . HoweverThe methods discussed above are used to prepare COFs. Nevertheless, the pure COFs usually have poor electrical conductivity, which limits their applications in the field of electrochemistry. Thus, new strategies that can improve the conductivity of COFs are urgently needed for their further electrochemical sensing applications.Combining COFs with some materials with specific conductivity, such as carbon materials, metal nanoparticles, metallic oxides and conducting polymers, can be considered an effective strategy to improve the electrochemistry performance of the COFs. Some of the strategies to enhance the COF electrical conductivity are described below :Carbon-based materials have good conductivity, high specific surface area, excellent mechanical strength and cost effectiveness . Thus, cp-Phenylenediamine and terephthalaldehyde, and then Ag nanoparticles were embedded into the COF. Finally, the Ag NPs-COF/GCE was obtained after the composite was deposited on the GCE surface. Due to the synergistic effect of the COF and AgNPs, the AgNPs-COF/GCE showed 1.7 and 1.5 times higher electrical conductivity than the bare GCE, COF/GCE, and AgNPs/GCE alone. A similar approach, described by Wu Yang et al. [The integration of metal nanoparticles (NPs) into a COF framework is another innovative method to improve COFs’ electrochemical performance. Not only can COFs provide multiple functional active sites and a large surface area for metal nanoparticles to integrate, but they can also prevent metal nanoparticle aggregation to obtain satisfactory dispersion. In addition, metal nanoparticles can improve the conductivity of COFs. Thus, the COF/metal nanoparticle composites have attracted more and more attention in the electrochemical field . One exag et al. , consist3O4 [2O4 [2O3 [3O4@TAPB-DMTP-COFs) by a step-by-step assembly method under facile conditions. The biggest peak current of luteolin on the TAPB-DMTP-COFs/GCE was observed on the Fe3O4@TAPB-DMTP-COFs/GCE .JinYu Qiao et al.  proposedThe above studies prove that a COF-based electrochemical sensor can achieve the determination of bisphenol compounds. However, the current reports mainly involve bisphenol A and bisphenol S determination. Hence, it is necessary to design a novel electrochemical sensor based on COFs to determine more kinds of bisphenol compounds.Antibiotics, as a class of pharmaceuticals that can inhibit or kill pathogens, are widely used to prevent and treat infectious diseases caused by bacteria, fungi, molds and other microorganisms . However3O4@COFs@MIPs) for the sensitive and rapid determination of TC. The Fe3O4@COFs@MIPs was synthesized according to a layer-by-layer modification method, which showed outstanding adsorption properties and good magnetism. The liner range of the electrochemical sensor for TC determination was 1 × 10−10–1 × 10−4 g/mL and the LOD was 2.4 × 10−11 g/mL (S/N = 3). The prepared sensor has also been successfully applied to milk and chicken samples. Xionghui Ma et al. [−6 μmol/L. In another example, Hongming He et al. [Tetracycline (TC) is a typical antibiotic that can cause resistance and other side effects such as allergic reactions, nephrotoxicity and liver impairment. At present, some electrochemical sensors based on COFs have been used for tetracycline antibiotic detection. For example, Yukun Yang et al.  proposeda et al.  construce et al.  synthesie et al. , sulfadie et al. , furazole et al. , penicile et al.  and chloe et al. .Tab-Dva nanofiber with excellent electroactivity for O, O-dimethyl-O-2, 2-dichlorovinylphosphate (DDVP) determination (Tab-Dva nanofibers can effectively immobilize acetylcholine and enrich thiocholine due to their large surface area and abundant vinyl. The LOD of the electrochemical biosensors was 0.11 μmol/L. More interestingly, the ratiometric electrochemical biosensor was successfully used to detect the DDVP residual in lettuce juice samples. Xianbo Lu et al. [Pesticide residues in food samples have become a great threat to human life. Thus, it is urgent to detect pesticide residues in a sensitive, selective, simple and rapid manner. Carbaryl is a carbamate pesticide that can cause damage to the nervous system and brain . Lili Chmination . COFTab-u et al.  also synu et al.  and Yongu et al.  have conAlthough there are few reports on the application of COF-based electrochemical sensors in pesticide residue determination, they still prove that this sensor is a promising tool for pesticide determination. Therefore, more research is urgently needed to expand the variety of COF composites and pesticides to validate these findings.2+, Hg2+, Cu2+, and Cd2+, can accumulate in the food chain at different stages. These heavy metals have serious effects on human health even at ultra-trace levels [Heavy metals, such as Pbe levels . Therefo2+ and Hg2+. The linear range of the sensor was 0.01–0.3 and 0.05–0.3 μmol/L for Pb2+ and Hg2+, respectively, with LODs of 0.72 × 10−3 and 1.2 × 10−2 μmol/L. Moreover, the electrochemical sensor was also used to detect Pb2+ and Hg2+ in edible samples. Madrakian et al. [3O4 nanoparticles. The glass carbon electrode successfully realized the selective detection of Pb2+ with a LOD of 0.95 nnmol/L. In order to achieve the simultaneous detection of multiple heavy metal ions, Yanyan Zhang et al. [2+, Pb2+ and Cd2+ due to the high enrichment capacity of the COF and the good conductivity of the MWCNTs. The LODs of the electrochemical sensor for Cu2+, Pb2+ and Cd2+ determination were 0.2, 0.7 and 0.4 μg/L, respectively.Tayyebeh Madrakian et al.  developen et al.  also prog et al.  reportedE. coli. The linear range of the electrochemical biosensor was 10–108 CFU/mL with the LOD of 3 CFU/mL.Aflatoxin is a metabolite of aspergillus flavus, aspergillus parasiticus and aspergillus norvegicus, which mainly exists in moldy peanuts, cereals and other foods . Due to Although only a few COF-based electrochemical sensors have been designed and applied to fungal toxin and bacterium analysis, it has still been proven that the COFs are a propagable material to construct an electrochemical sensor for fungal toxin and bacterium determination.Food safety is one of the critically important issues for human health with the continuous growth of food consumption and more reports of food contamination incidents. To ensure food safety, electrochemical sensors have been employed for food quality and safety testing due to their accurate, simple, cost-effective and fast response. Herein, we have reviewed the reported COF-based electrochemical sensors in the context of food safety analysis. Specifically, the design and synthesis methods of COFs, namely solvothermal synthesis, mechanochemical synthesis, solvent-free synthesis, microwave-assisted synthesis and sonochemical synthesis, have been summarized. Moreover, these reports have been discussed in terms of the strategies to improve the performance of COF based electrochemical sensors as well as their use in the determination of food contaminants, including bisphenols, antibiotics, pesticides, heavy metal ions, fungal toxins and bacterium. Thus, COFs provide an emerging platform for researchers to develop electrochemical sensors for food safety applications. Although COF-based electrochemical sensors have demonstrated excellent performance and promising prospects, there are still some challenges, such as the need to strengthen the stability and repeatability of the sensors and to realize the miniaturization and field operation of the sensors. Although challenges still exist, the exciting promise of these sensors in the electrical analysis of food will encourage researchers to conduct further research, combining new COFs with advanced electrochemical techniques to give electrochemical sensors perfect analytical performance.In general, the application of COF-based electrochemical sensors in the field of food analysis is still at an early stage, and there is still much room for research in this innovative field. Taking a future outlook, the rational design and controlled synthesis of multifunctional electrode materials based on COFs, while enhancing their selectivity to food contaminants, are very desirable for expanding their applications in the field of food analysis. In addition, the poor biocompatibility of COFs limits their potential application in contaminant detection in biological foods. In the future, the feasibility study of biocompatibility of electrochemical sensors based on COFs should be strengthened. The synthesis of new compatible/biocompatible COFs is very important for the analysis of contaminants in biological food. Furthermore, when detecting targets in complex systems, one of the main challenges faced by electrochemical sensors is the fouling of the electrode surface, which leads to the limited life of the sensors. This problem should be solved by designing and synthesizing COF electrode materials with antifouling performance.