silver island films were prepared by vacuum deposition of 7  nm silver onto glass coverslips . directly before use , films were annealed for 1  min at 300  c under argon . the sers substrates were immediately immersed in fresh 2  mm ethanolic thiol solutions for 1518  h. to alter the pntp ( fluka , technical grade ) surface concentration , the ratio between pntp and thiophenol ( merck , for synthesis ) was adjusted to keep the total thiol concentration constant . xray photoelectron spectroscopy measurements indicated no large deviation in thiol ratio once adsorbed onto the silver island film . sers spectra were measured through a 5 objective ( na 0.12 ) at 532  nm excitation with a power density of 610  w  cm by using a renishaw invia raman microscope . data processing was done with matlab  2013a and the baselinecorrected peak integration performed by using trapezoidal numerical integration . normalization of the total sers intensity was performed on the basis of the total sers intensity between 1020 and 1625  cm . a zeroorder savitzky  golay filter of 7 pixels was used to smooth the timedependent peak areas . the nonlinearmodelfitting function in mathematica  9.0 the sum of residuals and squares was calculated separately for both reactant and product , then added , to calculate r 
 .