the present invention is an electrochemical stripping sensor that employs colloidal gold modified electrode detection and square wave coulometric ( swc ) measurement of lead in blood , urine , water and other sample materials . the use of colloidal gold modified electrodes has been previously described in u . s . pat . no . 5 , 217 , 594 . the lead detection system of the present invention provides an easy - to - use , accurate and portable test system to detect lead levels in human blood , other body fluids , and environmental samples such as ground and surface waters . the fundamental technology of the square wave coulometric ( swc ) lead test involves specific electrochemical deposition of sample lead onto a colloidal gold modified carbon electrode , stripping the deposited lead , and coulometric analysis of the measured lead stripping current . the area - function that represents the lead stripping current is proportional to lead concentration and can be equated to lead in blood and other samples in μg / dl . the analytical challenge for a practical device to screen all children under the age of six for blood lead is met in a portable device by employing the colloidal gold modified sensor strip of the present invention . the system achieves accurate and precise results at current cdc guidelines for lead of 10 μg / dl and below . the present invention , in addition to being accurate , precise , portable and cost - effective , is relatively simple and easy - to - use so that individuals without extensive training obtain acceptable results in &# 34 ; real world &# 34 ; situations . the colloidal gold modified electrodes of the present invention require only a simple acid pre - treatment step for a blood sample prior to electrochemical analysis . it is possible for a relatively nontechnical personnel to handle a large number of samples in a relatively short period of time , thereby saving time and money . the system includes a reusable electronic meter and a square wave coulometric analysis protocol . the reusable electronic meter is a light - weight (˜ 1 lb ), durable , internally or externally powered electrochemical instrument . one selects a meter to display test results on an lcd screen and / or store data in internal memory . a convenient meter feature is an rs - 232 port so that stored data can be downloaded to a personal computer or network for further analysis or long - term storage . existing epidemiological information may also be stored in the meter &# 39 ; s internal memory for remote access and acquired data for epidemiological analysis can be downloaded for expanding shared databases . the square wave coulometric analysis protocol is programmed into the microprocessor contained in the reusable electronic meter . the dimensions of the meter are not critical and may be of a convenient size for lap - top or hand - held use . the disposable sensor strip configuration of the present invention is a small (˜ 1 inch ), rugged , printed , colloidal gold modified carbon stripping electrode . the foundation for the sensor strip is an inert polymer material . one first selects a large polymer sheet suitable for screen printing . this sheet may be screen printed with any array of carbon and silver 3 - electrode groupings . a dielectric material is next applied to protect the electrodes and form a recessed sample application area . colloidal gold is applied by controlled deposition techniques to the lead - sensing working electrode . the polymer sheet is then cut into discrete sensor strips , each sensor strip containing one 3 - electrode grouping , and packaged . the disposable colloidal gold modified sensor strip fits into a port on the reusable electronic meter in a manner to make electrical contact with the meter , and to keep the sample application area away from the meter to prevent contamination . acid treated sample ( e . g ., blood or other sample ) is applied to the sensing ( colloidal gold modified ) area of the electrode . the operator then pushes a button on the electronic meter and the swc analysis proceeds to completion in ˜ 2 minutes . the concentration of blood lead is displayed on the meter &# 39 ; s lcd . the blood contaminated sensor strip is removed from the meter and discarded in a safe manner . the following examples are included to demonstrate preferred embodiments of the invention . it should be appreciated by those of skill in the art that the techniques disclosed in the following examples represent techniques discovered by the inventors to function well in the practice of the invention , and thus can be considered to constitute preferred modes for its practice . however , those of skill in the art should , in light of the present disclosure , appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention . the colloidal gold modified electrodes of the present invention are prepared by controlled deposition of a colloidal gold solution onto a suitable substrate , such as glassy carbon or printed carbon ink . in certain modifications , the inventors have achieved excellent reproducibility by stabilizing colloidal gold sol solutions with a cationic polymer . the polymer apparently acts a dispersant as well as a stabilizer of the gold sol . an unexpected advantage of adding a polymer stabilizer is the elimination of background interference , possibly by selective membrane retention of interfering electrochemical species by the polymer . without the polymer , problems with interfering electrochemical species were frequently encountered . polymer addition may also lower the background current by blocking some of the conducting surface sites . typical preparations of a colloidal gold sensor electrode are described in this example . the disposable sensor strip is comprised of a three - electrode system . each electrode has a different function . the silver electrode provides an electrochemical reference during the measurement . the carbon electrode provides a counter pole for the working electrode . the colloidal gold modified carbon electrode is the working electrode and provides the lead deposition and stripping surfaces . these electrodes are prepared by screen printing three electrodes with conductive silver ink and then screen printing carbon ink over the counter and working electrode . the carbon working electrode is completed by the controlled volumetric deposition and evaporation of a known volume of colloidal gold sol / poly ( ester - sulfonic acid ) polymer mixture on the electrode surface . gold trichloride ( fisher chemical company , st . louis , mo .) was used to prepare a colloidal gold sol with a particle diameter of approximately 300 å by the method of moremans , et al , ( 1985 ). a solution of 1 % aqueous sodium citrate was added to a rapidly boiling , stirred solution of gold trichloride , and the solution was refluxed for 30 minutes . the final concentrations ( weight percent ) were 0 . 01 % haucl 4 and 0 . 03 % sodium citrate . gold trichloride ( fisher chemical company , st . louis , mo .) was reduced with ascorbic acid to prepare a colloidal gold sol with a particle diameter of approximately 500 å . the reaction was carried out at 0 ° c . and accomplished by the addition of ascorbic acid to a dilute solution of haucl 4 ( ratio 5 moles ascorbic acid per mole of haucl 4 ). the reaction solution is stirred for 90 minutes at 0 ° c . during the course of the reaction there was a gradual color shift from deep purple to red wine color indicating a maturation in particle size distribution . final size distribution of the colloidal gold did not occur for a least another 4 hours at room temperature . this method produced a colloidal gold suspension substantially equivalent to the citric acid reduction without using elevated temperatures . colloidal gold sols with particle diameters ranging from 100 å to 1000 å were purchased from bbi international , uk . dilute colloidal gold ( 0 . 05 mg / ml ) was concentrated 20 times by centrifugation for 30 minutes at 4000 rpm . poly ( ester - sulfonic acid ) polymer ( eastman chemical , kingsport , tenn .) was added to a final concentration of 0 . 9 % ( v / v ). three microliters of the ( 1 / ml ) colloidal gold sol / poly ( ester - sulfonic acid ) polymer solution was volumetrically deposited on the working electrode surface and spread to cover the entire exposed surface of the working electrode . the electrode was then dried at 30 °- 40 ° c . under a heat gun for 5 minutes . in this example , the working electrode surface was modified with 3 μg of gold . as an alternate to the above preparation , three microliters concentrated colloidal gold sol ( 1 mg / ml ) was volumetrically deposited on the working electrode surface and spread to cover the entire exposed surface of the working electrode . the electrode was then dried at 30 °- 40 ° c . under a heat gun for 5 minutes . two microliters of 2 % ( v / v ) poly ( ester - sulfonic acid ) polymer was volumetrically deposited onto the colloidal gold modified working electrode surface and dried for another 5 minutes . six water samples were tested . the samples were doped with lead to concentrations of 0 μg / dl , 0 . 5 μg / dl , 1 . 0 μg / dl , 2 . 0 μg / dl , 3 . 5 μg / dl , and 5 . 0 μg / dl respectively . the doped lead values were confirmed by anodic stripping voltammetry . carbon working electrodes were modified with 3 μl of 1 mg / ml 500 å colloidal particles or with 3 μl of 1 mg / ml 500 å colloidal particles with 1 % poly ( ester - sulfonic acid ) polymer as described in example 1 . water samples were pretreated with 100 mm hcl solutions . water ( 30 - 50 μl ) was placed on the disposable 3 - electrode sensor strip , described in example 1 , so as to cover all three electrodes within the circle of the insulating layer . a square wave coulometry ( swc ) program was initiated within 10 seconds of application of the sample on the disposable sensor strip . execution of the measurement sequence was confirmed by the monitor reading &# 34 ; test in progress &# 34 ;. within approximately 2 min . the display showed a lead concentration value . results for colloidal gold modified carbon electrodes are shown graphically in fig1 . results for colloidal gold particles with 1 % poly ( ester - sulfonic acid ) polymer modified carbon electrodes are shown graphically in fig2 . the colloidal gold modified carbon electrodes were coupled to a reusable electronic meter that provided the potentiostatic and swc analytical capabilities for the disclosed lead detection system . the meter can vary in size from hand - held to lap - top to table top dimensions . a block diagram of the reusable electronic meter &# 39 ; s components is shown in fig3 . conditioning 1 : a potential of - 500 mv ( vs . the silver reference electrode on the disposable sensor strip ) is applied to the strip &# 39 ; s working electrode ( colloidal gold modified carbon ) for 10 seconds . conditioning 2 : the potential is changed to - 50 mv and held at this value for 10 seconds . deposition : the potential is changed back to - 500 mv and held for 90 seconds . stripping and data acquisition : a square wave voltammetry scan is run from - 500 mv to + 50 mv , during which currents are measured ( twice during each square wave cycle ) and saved in the memory of the meter ( 552 measurements per scan ). the parameters of this square wave voltammetry are : frequency 50 hz , amplitude 25 mv , potential step 2 mv . data treatment : stored current values are numerically filtered ( forward and reverse and net currents are filtered individually ). a baseline subtraction algorithm is then executed and the lead stripping signal is measured by another algorithm that integrates the area under the stripping peak . the peak area value is then correlated with standard calibration data to determine the concentration of lead in the analyzed blood sample . the advantages of colloidal gold modified carbon electrodes compared with thin film ( bulk ) gold electrodes are not necessarily predicted or expected . the inventors have tested both types of electrodes in order to compare properties . the following describes direct comparisons of stripping voltammetry determinations of lead ion between colloidal gold modified carbon electrodes and bulk gold electrodes in the same samples and under the same experimental and instrumental conditions . in order to compare the sensitivity of a conventional , reusable , solid gold (&# 34 ; bulk gold &# 34 ;) disk electrode with the disclosed colloidal gold modified carbon electrode ( prepared with 1 . 0 mg / ml 500 å colloidal gold sol and 1 . 0 mg / ml 500 å colloidal gold sol mixed with 1 % poly ( ester - sulfonic acid ) polymer ), a series of parallel swc measurements of lead stripping response was conducted in 100 mm hcl solutions containing 0 - 5 mg / dl ( equivalent to 0 - 50 ppb ) of lead standard . such comparison can be meaningful only if the sensitivity is related to the active surface area of the compared electrodes . this can be done by either considering the geometric surface areas of the electrodes or considering the active surface areas of the electrodes . the former introduces an uncertainty related to the roughness of the electrode surface and the fact that the colloidal gold particles not in contact with the carbon support do not participate in the stripping process . the comparison of the sensitivity per active surface area is free of these uncertainties and therefore more meaningful . the bulk gold electrode had a diameter of 1 . 8 mm and a geometric surface area of 1 . 5 mm 2 . the screen printed carbon ink electrode had a geometric surface area of 14 . 6 mm 2 . the geometric surface area of the colloidal gold particles deposited on this carbon electrode was 18 . 7 mm 2 . the colloidal gold geometric surface area calculation was based on the number of colloidal gold particles ( determined from the measured average particle size ), colloidal gold concentration , assuming ideal sphericity of gold particles , and ignoring the roughness of the gold particle surface . the active electrode surface area was determined by measuring the charge required to reduce anodically oxidized gold from the electrode surface . this is a commonly accepted method of surface area determination for platinum and gold electrodes . a series of cyclic voltammetry ( cv ) experiments was conducted using each gold electrode and the same reference ( silver ink on the sensor strip ) and counter ( carbon ink on the sensor strip ) electrodes . solutions ( with ambient dissolved oxygen ) of 100 mm hcl were used . the charge was measured by integration of a noise - filtered and background - and baseline - subtracted reduction peak for au + 1 species generated during the 100 mv / s cv scan from 0 . 0 v to 1 . 2 v to - 0 . 5 v ( vs . ag reference electrode ). the measurements were repeated to assure reproducibility . the measured charge was then divided by 3 . 86 mc / mm 2 , which is the literature value of the standard charge per unit surface area for electrooxidation of gold . the surface area values obtained were 52 . 6 mm 2 for the bulk gold electrode and 2 . 6 mm 2 for the colloidal gold modified carbon electrodes . fig4 fig1 and fig2 show results of the swc lead measurement for , respectively , a bulk gold electrode , a colloidal gold modified carbon electrode , and a colloidal gold mixed with poly ( ester - sulfonic acid ) polymer modified carbon electrode . in these graphs the stripping signals are expressed in units of charge density , i . e . μc / cm 2 . these values were obtained by dividing the measured charge during the stripping scan by the active surface areas of the tested gold electrodes . the lead sensitivity of the colloidal gold modified carbon electrode ( slope of fig1 ) is approximately 5 . 6 times greater than the sensitivity of the bulk gold electrode ( slope of fig4 ). the lead sensitivity of the colloidal gold mixed with poly ( ester - sulfonic acid ) polymer modified carbon electrode ( slope of fig2 ) is approximately 14 . 5 times greater than the sensitivity of the colloidal gold modified carbon electrode ( slope of fig1 ) and 81 times greater than the sensitivity of the bulk gold electrode ( slope of fig4 ). synthetic urine is an aqueous solution of salts and organic compounds that mimics the chemical and physical properties of human urine . this product was purchased from cst technologies , inc . of great neck , n . y . five urine samples were tested . the samples were doped with lead to concentrations of 1 . 0 μg / dl , 2 . 0 μg / dl , 4 . 0 μg / dl , 8 . 0 μg / dl , and 16 μg / dl respectively . the doped lead values were confirmed by anodic stripping voltammetry . carbon working electrodes were modified with 3 μl of 1 mg / ml 500 å colloidal particles with 1 % poly ( ester - sulfonic acid ) polymer as described in example 1 . samples were pretreated with 100 mm hcl solutions . sample ( 30 - 50 μl ) was placed on the disposable 3 - electrode sensor strip , described in example 1 , so as to cover all three electrodes within the circle of the insulating layer . the square wave coulometry ( swc ) program was initiated within 10 seconds of application of the sample on the disposable sensor strip . execution of the measurement sequence was confirmed by the monitor reading &# 34 ; test in progress &# 34 ;. within approximately 2 min . the display showed a lead concentration . results are shown graphically in fig5 . the reusable electronic meter &# 39 ; s microprocessor automatically executes the sequence described in example 2 . this example illustrates the rapid and accurate determination of lead in blood samples . the inventors have discovered that pretreatment of blood samples to bring the ph below 1 without significant dilution is important in preventing polymerization and coagulation of blood components . a suitable pretreatment was achieved using 400 mm hydrochloric acid . blood samples were analyzed shortly after being drawn ; alternatively , they were refrigerated for several days before analysis . for longer storage ( e . g ., longer than a week ) blood samples were frozen . frozen blood samples including those used as blood lead reference materials ( e . g ., centers for disease control standards ), should be thawed and allowed to stand at least 30 minutes at room temperature before analysis . three volumes of 400 mm hcl solution was added to 1 volume of blood in a disposable 1 . 5 ml microcentrifuge tube . the tube was capped and immediately stirred vigorously by turning the tube upside - down at least 5 times with tapping between turns . after 5 minutes the amount of lead in the sample was determined using square wave coulometry as described in example 2 . three different blood samples were tested with disclosed lead sensor system , bovine blood lead control samples prepared by the cdc , freshly drawn human blood , and three commercially available whole human blood lead control samples obtained from utak labs , inc . ( valencia , calif .). the cdc bovine samples contain ingested lead and were sent with target values . the freshly drawn human blood was spiked with lead after taking the blood sample . the utak human control samples were doped with lead and had assigned lead concentrations of 9 μg / dl , 22 μg / dl and 34 μg / dl respectively , the cdc and utak assigned values were confirmed by graphite furnace atomic absorption spectrometry and anodic stripping voltammetry . the freshly drawn human blood values were confirmed by anodic stripping voltammetry . the carbon working electrode was modified with 3 μl of 1 mg / ml 200 å colloidal gold particles with 0 . 5 % poly ( ester - sulfonic acid ) polymer as described in example 1 . blood samples were pretreated with hcl solution as described in example 5 . blood ( 30 - 50 μl ) was placed on the disposable 3 - electrode sensor strip , described in example 1 , so as to cover all three electrodes within the circle of the insulating layer . the square wave coulometry ( swc ) program was initiated within 10 seconds of application of the sample on the disposable sensor strip . execution of the measurement sequence was confirmed by the monitor reading &# 34 ; test in progress &# 34 ;. within approximately 2 min . the display showed a lead reading . results for the bovine blood samples are shown graphically in fig6 . results for the freshly drawn human blood samples are shown graphically in fig7 . results for the utak human control blood samples are shown graphically in fig8 . the reusable electronic meter &# 39 ; s microprocessor automatically executes the sequence described in example 2 . this example illustrates the rapid and accurate determination of lead in blood samples with 500 å colloidal gold particles deposited on screen printed carbon electrodes covered with a woven screen mesh . the inventors have found that a hydrophilic material covering the three electrode sensor , such as described in example 1 facilitates the rapid and uniform distribution of a blood sample over the desired area . a suitable hydrophilic material is a polyester monofilament fabric purchased from tetko , inc . ( briarcliff manor , n . y .). a colloidal gold modified carbon electrode was fabricated as described in example 1 with 500 å colloidal gold particles . a woven screen mesh of polyester monofilament fabric was affixed over the sample placement area of the working electrode , reference electrode , and counter electrode . mesh covered colloidal gold modified electrodes were used to test three different blood lead control materials purchased from utak labs , inc . ( valencia , calif .) that were acid treated as described in example 5 . these electrodes measured lead values for these control samples that were on average within 7 . 8 % of their assigned lead values . representative data are shown in fig9 . a reusable electronic meter &# 39 ; s microprocessor automatically executed the sequence described in example 2 . this example illustrates certain electrochemical properties of the colloidal gold modified electrodes developed by the inventors . the colloidal gold modified electrodes act as a microelectrode array . for the same geometric area , the stripping detection signal of a microelectrode array is greater than the stripping detection signal for a planar electrode . the response of microelectrode arrays has three regimes of scanning frequency dependence . the divisions between the regimes depends on the analyte diffusion distance relative to the radius of the individual microelectrodes and the separation between the individual microelectrodes . in high frequency scans , analyte diffusion distance is small compared to microelectrode radius . signal is limited by the aggregate area of the microelectrodes and is proportional to the square root of frequency . in low frequency scans , analyte diffusion distances to the individual microelectrodes overlap . signal is limited by the macroscopic geometric area of the array and is proportional to the square root of frequency . in between frequency scan extremes analyte diffusion distances are not small compared to microelectrode radius but do not overlap the individual microelectrodes . signal response is in a steady - state and essentially is independent of frequency . signal response exhibits mixed behavior at the boundaries of these regions . the frequency positions of the boundaries give information about the average microelectrode radius and the average distance of separation of the microelectrodes . at a frequency of 1 hz the disclosed colloidal gold modified electrode exhibited a steady - state response to 1 ppm lead that extends at least from 0 . 5 to 2 hz . exploration of the steady state frequency boundaries of the disclosed colloidal gold modified electrodes provided an to estimate of microelectrode size ( radius = 1 . 5 × 10 - 3 cm ), microelectrode number ( 9 × 10 4 ), and distance of separation of the microelectrodes ( 1 . 2 × 10 - 3 ). the sensitivity of the electrode under the instrumental conditions used in this example is represented by : after correction for geometric areas , the relative signal response of colloidal gold modified carbon electrodes in unstirred samples was ≧ 2 the relative signal response of bulk gold electrodes in unstirred samples . additional evidence to support the microelectrode array behavior of the disclosed colloidal gold modified carbon electrode was obtained by examining the dependence between the swc signal and the deposition time in the absence of forced convection , e . g . stirring . the swc signal for blood lead , or the stripping peak current , increased in proportion to the increase of the deposition time . this linear dependence proved the nonlinear diffusion pattern of the transport of lead ions to the electrode surface during the deposition step , which is characteristic of the microelectrode array behavior . a square root dependence between the signal and the deposition time would be expected for a macroelectrode in a quiescent solution . furthermore , it was observed that under the conditions of relatively fast swv the disclosed colloidal gold modified carbon electrodes generated sigmoidal shaped forward and reverse current curves with a diffusion plateau well above that expected for a macroelectrode . forward and reverse current curves obtained in swv stripping experiments involving lead and the disclosed colloidal gold modified electrodes , exhibit a sigmoidal - like shape with a diffusion plateau well above . this shows that under the conditions of relatively fast swv the disclosed electrodes acted as a microelectrode array . all of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure . while the compositions and methods of this invention have been described in terms of preferred embodiments , it will be apparent to those of skill in the art that variations may be applied to the composition , methods and in the steps or in the sequence of steps of the method described herein without departing from the concept , spirit and scope of the invention . more specifically , it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved . all such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit , scope and concept of the invention as defined by the appended claims . the following references , to the extent that they provide exemplary procedural or other details supplementary to those set forth herein , are specifically incorporated herein by reference . biberian , j . p . and rhead , g . e . ( 1973 ) j . phys . f ( metal phys . ), 3 , 675 . copeland , t . r . and skogerboe , r . k . ( 1974 ) anal . chem ., 46 , 1257a . lord jr ., s . s ., o &# 39 ; neill , r . c ., and rogers , l . b . ( 1952 ) anal . chem ., 24 , 209 - 213 . moremans , m ., daneels , g . and demay , j . sensitive colloidal metal ( gold or silver ) staining of protein blots on nitrocellulose membranes . anal . biochem ., 145 , 315 - 321 , ( 1985 ). perdereau , j ., biberian , j . p . and rhead , g . e . ( 1974 ) j . phys . f ( metal phys . ), 4 , 798 . sawyer , d . t . and roberts , jr ., j . l . &# 34 ; experimental electrochemistry for chemists ,&# 34 ; wiley , 1974 wang , j ., stripping analysis : principles , instrumentation , and applications ; vch publishers ; deerfield beach , fla ., 1985 . wojciechowski , m . ; balcerzak , j . square - wave anodic stripping voltammetry at glassy - carbon - based thin mercury film electrodes in solutions containing dissolved oxygen . anal . chem ., 62 , 1325 - 1331 , ( 1990 ).