with the growing focus 
 on charge transfer and storage applications 
 using nanostructures , a fundamental understanding of the physical and chemical processes 
 governing charge transfer at the nanoscale is of paramount importance . because the catalytic properties of nanoparticles 
 are derived from their ability to store and transfer charge , nanoelectrodes prepared by attaching metal nanoparticles at submonolayer 
 coverage on conductive substrates can be used to experimentally test 
 the catalytic properties of nanoparticles . electrochemical methods 
 afford unrivaled control of surface chemistry at metal electrodes 
 but are classically built upon bulk electrochemical current , potential , 
 and charge relationships . changes in 
 surface charge density of gold nanoparticles can also be detected 
 through changes in the surface plasmon resonance energy . the use of these spectral characteristics to infer electrochemical 
 processes is the subject of nanoparticle plasmon spectroelectrochemistry . nanoparticle size and morphology have proven to be critical in 
 the design and engineering of efficient nanocatalysts . chemically prepared nanoparticles are inherently heterogeneous in 
 size and shape even under identical growth conditions , leading to 
 heterogeneity in catalytic and electrochemical activity within nanoparticle 
 populations . the synthesis of nanocatalysts has historically relied heavily on 
 iterative trial and error optimization schemes . several groups have recently taken advantage of theoretically 
 predicted shape - dependent activity in their quest to make better catalysts . if scientists and engineers could 
 resolve heterogeneous activity within populations of nanoparticles , 
 the study of highly active subpopulations could then be used to inform 
 design principles and further optimize catalytic properties of entire 
 populations , increasing total activity and yield . single - particle 
 surface plasmon sensing of catalytic and electrochemical 
 charging was first demonstrated by the mulvaney group and later reported by the klar 
 group with a focus on the ability to 
 tune the plasmon resonance of single nanoparticles with static potential 
 control . a related technique utilizing the surface plasmon resonance 
 of thin films has also been used to study electrocatalytic behavior 
 of adsorbed single nanoparticles and the metal film itself . hill and pan recently demonstrated the use of single - particle dark - field 
 spectroscopy in tracking the synthesis of single silver nanoparticles 
 via electrodeposition . without the deleterious 
 effects of inhomogeneous spectral broadening due to sample inhomogeneity 
 as in ensemble measurements , surface plasmon resonance and homogeneous 
 line width responses directly probe changes in free electron density 
 and plasmon damping due to surface chemistry . according to the charge density tuning model , 
 changes in charge density affect the bulk plasma frequency of the 
 metal , resulting in shifts of the surface plasmon resonance . however , recent ensemble spectroelectrochemical studies suggest 
 that chemical mechanisms might play an equal or more important role 
 in electrochemical tuning of the surface plasmon resonance . we hypothesize that because nanoparticle size and surface heterogeneity 
 are important in charge transfer and storage at nanoparticle interfaces , 
 disparities between the charge density and chemical tuning interpretations 
 can be addressed by resolving nanoparticle heterogeneity . ideally , one would want to statistically sample 
 the overall nanoparticle population but also have the ability to study 
 subpopulations at the single - particle level if multiple mechanisms 
 are identified . we have developed a methodology and complementary 
 techniques to 
 study electrochemical heterogeneity both of a statistical distribution 
 and within subpopulations of chemically prepared single gold nanoparticles . 
 by combining dark - field hyperspectral imaging and short exposure single - particle 
 spectroscopy under electrochemical control , we probed electrochemical 
 processes of single 50 nm gold colloidal particles over a broad range 
 of time scales and over two distinct potential ranges . previous studies 
 have focused on a few selected single nanoparticles measured serially 
 and are thus blind to heterogeneous behavior across nanoparticle populations , 
 particularly with regard to irreversible processes . because our methodology 
 measures many nanoparticles in parallel , we identify and illuminate 
 the heterogeneous nature of electrochemical plasmon resonance tuning 
 that has not been shown in existing literature . we identified three different subpopulations of nanoparticles in 
 the cathodic potential range on the basis of their spectral response 
 and concluded that previous studies involving single - particle spectroelectrochemistry 
 focused on the least populous group . by extending our dynamic spectroelectrochemical 
 investigation of these nanoparticles into the anodic potential range , 
 we observed multiple nanoparticle - dependent spectral tuning mechanisms . 
 in light of the multiple mechanisms and processes identified in this 
 system , we report on both the classification of the broader processes 
 present and further study of electrochemically reversible processes . the process - specific activity of individual nanoparticles shown clearly 
 in this work demonstrates the value of the methodology and technique , 
 and findings herein pave the way toward further understanding of nanoparticle 
 electrochemistry critical for guiding the design of nanoparticle electrodes 
 and catalysts . gold spheres 
 were purchased from bb international ( nominal size : 
 50 nm ; independent transmission electron microscopy characterization : 
 51  7 nm ) and deposited on an indium 
 tin oxide ( ito ) coated glass coverslip ( fisher scientific ) . a sealed 
 spectroelectrochemical cell for transmission dark - field microscopy 
 was constructed in - house using the ito / gold nanoparticle working electrode , 
 silver wire quasi - reference electrode , and silver wire auxiliary electrode . single - particle scattering 
 spectra were collected with a custom instrument comprised of an inverted 
 dark - field microscope ( zeiss axioobserver m1 , with oil immersion dark - field 
 condenser na = 0.71.4 ) , an electrochemical workstation ( ch 
 instruments , model 630d ) , and an imaging spectrograph ( princeton instruments , 
 acton spectrapro 2150i with pixis 400 thermoelectrically cooled back - illuminated 
 ccd ) mounted atop a programmatically controlled linear translation 
 stage ( newport linear actuator model lta - hl ) . the potential of the 
 nanoparticles relative to the silver quasi - reference electrode in 
 100 mm nacl electrolyte was controlled using a three - electrode potentiostat 
 synchronized with the spectrograph by means of custom software ( labview , 
 2011 ) . the scattered light was collected by an oil immersion 
 objective ( zeiss , plan - achromat 63x , na 0.71.4 ) and directed 
 to an imaging spectrograph to record scattering spectra of single 
 gold nanoparticles during cyclic voltammetry and chronocoulometry . 
 using a nonlinear least - squares fitting algorithm ( matlab , 2013a ) , scattering spectra were fit with single lorentzian curves , from which 
 the plasmon resonance energy and the full width at half - maximum were 
 found . our custom setup allows hyperspectral 
 imaging of many nanoparticles 
 and single - particle cyclic coulometry with specified potentials , realized 
 for both cathodic and anodic conditions ( figure 1 ) . a sealed optically transparent thin - layer electrochemical cell 
 with 50 nm gold nanoparticles dispersed on the working electrode forms 
 a three - electrode cell with a 100 mm sodium chloride ( nacl ) electrolyte 
 ( figure 1a ) . silver wires were used as an auxiliary 
 electrode and a quasi - reference electrode . a three - electrode potentiostat 
 controlled the potential difference at the interface of the electrolyte 
 and the gold nanoparticle / indium tin oxide ( ito ) working electrode . detailed information on the construction of the electrochemical cell 
 and experimental details can be found in section 1.1 of the supporting information . the electronic free charge 
 densities of the gold nanoparticles changed to establish each prescribed 
 potential difference and were manifested as shifts in the localized 
 surface plasmon resonance . resonantly scattered light from individual 
 nanoparticles was resolved with a 63 oil - immersion objective 
 and directed to a thermoelectrically cooled back - illuminated ccd imaging 
 spectrograph . by using a very thin spectroelectrochemical cell ( total thickness : 
 1 mm ) with nanoparticles directly adjacent to the collection optics , 
 out of focus light was minimized , leading to high signal - to - noise 
 ratios ( snrs  425 ) of the dark - field scattering signal . in 
 combination with lorentzian fitting , our instrumentation and experimental 
 geometry allowed us to measure shifts in single - particle plasmon resonances 
 as small as 1 mev within a few seconds . the spectrograph was mounted 
 atop a linear translation stage , and both the spectrograph and stage 
 were programmatically controlled and synchronized to construct hyperspectral 
 images of sample regions containing many nanoparticles ( instrument 
 details are provided in the supporting information sections 1.3 and 1.4 ) . using a pushbroom hyperspectral imaging scheme , tens to hundreds of single - particle spectra were acquired in parallel 
 under potential control . in figure 1b , a representative 
 50  50 pixel rgb image of ito - supported gold nanospheres the rgb image was reconstructed from a hyperspectral data cube , 
 shown in cartoon form behind the image . details for rgb construction 
 from hyperspectral data can be found in section 1.5 of the supporting information . collection of a hyperspectral 
 image at a fixed electrochemical potential took 108 s ( 2 s / pixel row , 
 54  284 pixel image ) with an additional 60 s of equilibration 
 time between scans at two different static potentials . in this manner , potential - dependent spectral traces from each nanoparticle were collected 
 as a series of hyperspectral data cubes , each at a prescribed electrochemical 
 potential . two example potential controlled spectra from a single 
 nanoparticle are shown in the right panel of figure 1b . ( a ) optically transparent thin electrochemical cell for dark - field 
 spectroscopy of single 50 nm gold spheres on an ito working electrode 
 ( we ) under electrochemical potential with auxiliary and reference 
 electrodes ( ae , re ) composed of silver wires . ( b , left ) steady - state 
 hyperspectral imaging of many single nanoparticles under potential 
 control ( 32  32 m ) . ( b , right ) normalized spectra at 
 potential vertices for a single nanoparticle along with lorentzian 
 fits . ( c ) non - faradaic cathodic ( blue ) and anodic ( red ) potential 
 ranges with predicted charging mechanisms shown schematically . using the above setup , single - particle 
 spectra were collected under 
 two potential ranges ( figure 1c ) . in the cathodic 
 range ( u < 0 v , shown in blue in figure 1c and throughout this work ) , electrons were injected 
 into each nanoparticle from the conductive ito substrate . the nanoparticles had a net negative charge , 
 corresponding to a higher free electron density than a neutral nanoparticle . 
 according to the grahame model for electrode / electrolyte contact , 
 a positively charged stern layer composed primarily of hydrated cations 
 forms at the surface of the nanoparticles as well as at the surface 
 of the ito ( figure 1c ) . in the anodic range ( u > 0 v , shown 
 in 
 red in figure 1c and throughout this work ) , 
 electrons are extracted from the nanoparticles by the potentiostat 
 circuit , yielding lower free electron densities and net positively 
 charged nanoparticles . to screen the positively charged gold surface , 
 hydrated chloride anions form a negatively charged stern layer . the two potential ranges were arbitrarily defined 
 with 0 v ( vs ag / agcl quasi - reference electrode ) as a common vertex 
 as a matter of convention set by seminal work . in the cathodic potential range investigated ( 0 to 
 800 
 mv ) , the only known faradaic process for gold electrodes in a nacl 
 solution is the overpotential deposition of hydrogen cations and subsequent 
 hydrogen evolution reaction ( her ) . these coupled reactions are unavoidable 
 at cathodic overpotentials and have been shown to rely heavily on 
 crystal lattice structure and the presence of defects . protonation of the gold surface is electrochemically reversible , 
 whereas her is electrochemically and chemically irreversible . as such , 
 we expect nanoparticle heterogeneity to result in a variety of observed 
 spectral tuning behaviors in the cathodic potential range . however , 
 existing literature reports only on nanoparticles following the charge 
 density tuning model . in the anodic potential range 
 ( u > 0 v ) , multiple reactions occur at gold electrodes 
 in nacl solutions . with a ph - neutral 
 nacl solution , the lowest potential anodic oxidation reaction is that 
 of chloride ions , which occurs at 530 mv ( vs ag / agcl reference 
 electrode ) , as detected in ensemble spectroelectrochemical measurements . in an attempt to avoid this reaction and target 
 charge - density - related surface plasmon resonance shifts , we limited 
 our upper potential vertex to + 400 mv . because of the above - mentioned 
 ( and other possible ) coupled reactions , we examined the cathodic and 
 anodic regimes separately . potential - dependent hyperspectral imaging gave us the means to 
 observe heterogeneity within a nanoparticle population ( figure 2 ) . hyperspectral images at each potential were used 
 to identify the steady - state potential induced plasmon resonance shift . figure 2a shows an example spectral image compiled 
 at open - circuit potential , with circled nanoparticles noting subsets 
 identified by their potential - dependent spectral behavior . each scattering 
 center in the series was located and fit with a single lorentzian 
 function . single gold nanoparticles were distinguished from clusters 
 by imposing a coefficient of determination cutoff , r > 0.95 . because we achieved a large signal - to - noise 
 ratio , this allowed us to reliably and repeatedly 
 detect spectral shifts as small as 1 mev . cathodic hyperspectral 
 steady - state electrochemical tuning of gold 
 nanoparticles on ito support . ( a ) real - color rgb image of 50 nm gold 
 nanoparticles at open - circuit potential . ( b ) classification of single - particle 
 behavior based on the reversibility of potential - dependent spectral 
 changes . ( c ) scattering spectra at potential vertices for a single 
 nanoparticle from subgroup reaction one . ( e ) spectra for a nanoparticle showing 
 charge density tuning of the plasmon resonance . ( f ) mean peak resonance 
 shift and associated standard error for seven nanoparticles exhibiting 
 charge density tuning , shown as a function of applied potential . arrows 
 indicate applied potential scan directions . by investigating many nanoparticles under potential control 
 in 
 this cathodic range ( 0 to 800 mv ) , we found that the majority 
 of nanoparticles demonstrated behavior not predicted by charge density 
 tuning , as shown in figure 2b . we expected 
 the plasmon resonance of all gold nanoparticles to blue shift linearly 
 upon application of negative electrochemical potentials , due to an increase in free electron density that results from electrons 
 flowing from the potentiostat circuit into the nanoparticles and substrate 
 in establishing the electrical potential difference between the bulk 
 electrolyte and the nanoelectrodes . upon exposure to cathodic potentials , 
 spectra for half of the single gold nanoparticles that met the selection 
 criteria ( r > 0.95 ) showed significant 
 irreversible changes in their scattering spectra including significant 
 resonance broadening , large increases in scattering intensity , and 
 the loss of their original lorentzian response . these nanoparticles 
 are highlighted with red circles in figure 2a , and representative spectra at the potential vertices are shown 
 in figure 2c . further identified in 
 figures 2a and 2b , one - fourth 
 of the nanoparticles also showed large 
 increases in intensity , spectral broadening , and plasmon resonance 
 red shifts not predicted by the charge density tuning model . in contrast 
 to nanoparticles undergoing reaction one , the scattering spectra of 
 nanoparticles in this subset maintained their lorentzian line shape 
 throughout the entire experiment , but their scattering spectra only 
 returned to initial conditions after the application of a sufficient 
 positive potential . these nanoparticles are highlighted by cyan circles 
 in figure 2a , and example spectra are shown 
 from a single nanoparticle in figure 2d . an 
 example of this apparent reaction , referred to as reaction two , is 
 shown in section 2.1 of the supporting information . further investigations are required to elucidate the mechanisms 
 and potentially complex behaviors that we refer to collectively as 
 reactions 1 and 2 . finally , just under a quarter of the single 
 gold nanoparticles 
 followed the predicted charge density tuning model . these nanoparticles 
 are indicated by green circles in figure 2a , 
 and example spectra from a nanoparticle in this subset are shown in 
 figure 2e . nanoparticles in this subset were 
 well - fit throughout the experiment ( r > 0.95 ) , show small changes in fwhm (  < 20 mev ) , 
 and demonstrated completely reversible plasmon resonance shifts . the 
 change in peak resonance energy as a function of potential for this 
 subset of nanoparticles is shown in figure 2f as a mean resonance shift with associated standard error for all 
 nanoparticles in this subpopulation . the return to initial resonance 
 energy is a strong indicator that the spectral tuning mechanism for 
 this subset of nanoparticles is electrochemically reversible , fitting 
 the charge density tuning model . by design , this experiment observed 
 the initial response of particles to electrochemical modulation . the 
 relatively large populations of particles demonstrating potential - dependent 
 spectral responses other than those predicted by the charge density 
 tuning model indicate that future work will be necessary to model 
 and fully understand the irreversible and semi - irreversible spectral 
 tuning observed here in the cathodic potential range . previous studies 
 of single - particle spectroelectrochemical tuning under static potential 
 control have only reported on the behavior of this particular subset 
 of nanoparticles . the most valuable electrochemical 
 techniques are not steady - state 
 techniques but rather are dynamic in nature . the ability to precisely and quickly vary the potential at an electrode 
 surface allows researchers to characterize electrodes , investigate 
 fine potential structure , and determine reaction kinetics . for the 
 same purpose , in this work we developed dynamic single - particle spectroelectrochemistry . 
 by measuring single - particle spectra under dynamic potential control , 
 we recorded spectral changes on the order of seconds rather than minutes 
 as with steady - state experiments . while the value of hyperspectral 
 potential - controlled imaging experiments lies in studying many nanoparticles 
 simultaneously , the strengths of dynamic measurements are 3-fold : 
 ( 1 ) fast collection times allow many potential cycles to be measured , 
 allowing us to probe the repeatability and reversibility of potential - dependent 
 spectral shifts . ( 2 ) by investigating the potential scan - rate dependence 
 of spectral response , the time scale of reaction kinetics can be interrogated . ( 3 ) reactions with partial chemical reversibility can be probed by 
 limiting the amount of time spent with significant electrochemical 
 overpotentials . from a practical standpoint , recording spectra on 
 the time scale of seconds vs minutes allows greater potential resolution 
 to be achieved in a realistic time frame . therefore , dynamic measurements 
 illuminate fine potential / resonance structure not possible using static 
 measurement techniques . single - particle measurement of dynamic electrochemical 
 tuning in 
 the cathodic range ( 0 to 800 mv ) . ( a ) electrochemical potential 
 of the working electrode relative to the reference electrode as a 
 function of time . sweep rate : 10 mv / s . the fitted resonance energy , eres ( solid white line ) , and full width at half - maximum , 
  ( dotted white lines ) , are superimposed . ( c ) reversible , cyclic eres response as a function of potential u shown as mean with standard error for three consecutive 
 cycles . ( e ) mie  drude model simulations , and the number of electrons 
 transferred from the conductive substrate to the nanoparticle is calculated . investigations under dynamic potential 
 control in the cathodic 
 potential range showed that the subset of nanoparticles described 
 in figure 2e and 2f 
 also demonstrated reversible spectral tuning , consistent with the 
 charge density tuning model ( figure 3 ) . in 
 this experiment , the potential was swept in a sawtooth pattern at 
 10 mv / s between 0 and 800 mv , as shown in figure 3a . scattered light from a single nanoparticle was 
 directed to the spectrograph , and spectra were recorded every 2.5 
 s. each spectrum was independently fit with a single lorentzian , and 
 the parameters of the fit determined the resonance energy , eres , and full width at half - maximum , . eres and  are shown in figure 3b as solid and dashed white lines , respectively . the mean and standard error for eres and  for three consecutive potential cycles after initial 
 stabilization are shown as a function of applied electrochemical potential 
 in figures 3c and 3d , 
 respectively . eres blue - shifted linearly 
 with increasingly negative potentials and returned to the initial 
 resonance as the potential u was swept back to 0 
 v ( deres / du = 3.9 
 mev / v ) . a small but measurable potential - independent hysteresis is 
 also apparent . to test whether this hysteresis was related to double - layer 
 formation kinetics , the scan rate was varied ( experimental data for 
 5 , 10 , and 20 mv / s is given in section 2.2 of the supporting information ) , but no effects due to changes in 
 scan rate were observed . we conclude that because no scan rate dependence 
 was found for these values transient cell and double - layer dynamics 
 occurred faster than the rate of potential changes , as expected from 
 theory . by corollary , observed spectral 
 shifts for these nanoparticles in this potential range probed changes 
 in equilibrium charge densities . these results are in good qualitative 
 agreement with the charge density tuning model . using the charge - density - modified 
 drude dielectric function model 
 along with mie scattering theory ( mie  drude model ) , we calculated the excess charge on the nanoparticle 
 from eres throughout the potential range 
 applied in our study ( figure 3e ) . in these calculations , a model drude dielectric 
 function was modified to reflect changes in the nanoparticle free 
 electron density and then used to simulate the plasmonic response 
 as a function of applied electrochemical potential . details and derivation 
 of the full mie  drude model are found in section 3 of the supporting information . at negative potentials , the nanoparticle was negatively charged and therefore had a higher 
 free electron density , as shown in figure 3e . over this potential range the number of excess electrons fluctuated 
 by 20  000 electrons , which is still less than 0.5% 
 of the total conduction electrons in the nanoparticle but corresponded 
 to a surface capacitance of 510 f / cm . figure 3d shows that  decreased linearly with applied 
 potential , with d/du = 3.2 mev / v . however , 
 the trends in  and scattering intensity predicted by the mie drude 
 model scattering simulations were completely opposite from the experimental 
 results ( for more details see section 3 of the supporting information ) . for this reason , we suggest that 
 significant theoretical work is still needed to unravel the dependence 
 of resonance width and intensity on applied potential . in the calculation 
 of free charge density fluctuation , each nanoparticle was assumed 
 to be electrically neutral at the open - circuit potential of the working 
 electrode . the actual point of zero charge for each nanoparticle is 
 unknown and likely varies slightly from nanoparticle to nanoparticle . it should be noted that previous studies have confirmed that changes 
 in refractive index do not play a significant role in electrochemically 
 induced plasmon resonance tuning by varying the cationic species and 
 observing no change in cathodic charge density tuning response . from a survey of 13 nanoparticles in this potential 
 range obeying the charge density tuning model , we found that the response 
 was linear but deres / du varied greatly among nanoparticles from 2 to 16 
 mev / v , indicating that the nanoparticles ability to store 
 charge varied or that heterogeneity between the conducting substrate 
 and metal nanoparticles played a limiting role in overall charging . 
 for these nanoparticles , when the nanoparticles found to undergo 
 charge density tuning at 
 cathodic conditions were examined within the anodic range with dynamic 
 potential control , a heterogeneous spectral response was observed 
 ( figure 4 ) , indicative of multiple processes 
 occurring . in the anodic potential range of 0 to + 400 mv , well below 
 the chloridation onset potential expected from ensemble measurements 
 ( 530 mv vs ag / agcl ) , some nanoparticles 
 exhibited nonlinear spectral tuning , inconsistent with the charge 
 density tuning model . figure 4a illustrates 
 a series of spectra as a function of time varying potential for two 
 separate nanoparticles demonstrating the two observed behaviors . the electrochemical potential was again swept 
 at 10 mv / s between 0 and 400 mv as shown in figure 4b . take note that large modulations of the scattering intensity 
 with potential produce an illusion of resonance narrowing at higher 
 potentials , when in fact the opposite is true increases 
 with increasing potential . figure 4c plots 
 the plasmon resonance energy as a function of potential averaged over 
 three consecutive cycles . the resonance energy decreased linearly 
 with increasing potential as predicted by the charge density tuning 
 model ( deres / du = 7.3 
 mev / v ) . again , in figure 4d ,  increases 
 linearly in contrast to predictions by the mie  drude model 
 scattering simulation for charge density depletion ( d/du = 10.6 mev / v ) . using this model and applying it to the 
 change in resonance energy ( figure 4c ) , the 
 nanoparticle s net electron flux is reported as a function 
 of potential in figure 4e but needs to be treated 
 with care for the reasons already given . from the 13 nanoparticles 
 investigated in the anodic potential range , all of them underwent 
 reversible plasmon resonance shifts , but only 8 nanoparticles displayed 
 linear resonance shifts and broadening with potentials like particle 
 1 . d/du for these nanoparticles varied from 2 to 32 mev / v , nearly 
 3 times larger than seen in the cathodic range . we attribute this 
 increase in resonance broadening to interface damping , resulting from 
 the much higher polarizability of chloride ions compared to sodium 
 ions . the other 5 nanoparticles showed 
 a nonlinear plasmonic response to the applied electrochemical potential 
 ( particle 2 shown in figure 4 ) and will be 
 discussed next . ( a , f ) 
 scattering 
 spectra collected during dynamic potential control shown with eres and  superimposed with solid and 
 dashed white lines , respectively . ( b ) electrochemical potential of 
 the working electrode as a function of time . ( c , g ) average eres over three consecutive 
 cycles shown as a function of potential . ( d , h ) average  is 
 shown as a function of potential . ( e , i ) the number of electrons transferred 
 from the nanoparticle to the substrate is shown as a function of potential , 
 calculated using the mie scattering spectra for particle 2 are shown in 
 figure 4f under the potential modulation given 
 in figure 4b . figure 4 g illustrates the mean and standard error for eres as a function of potential over three consecutive 
 cycles after initial stabilization . in contrast to particle 1 , eres for particle 2 decreased nonlinearly with 
 increasing potential in the anodic range and exhibited two distinct 
 resonance vs potential slopes . from 100 to 220 mv , deres / du = 23.2 mev / v , nearly six 
 times larger than the linear shift observed for the nanoparticle in 
 the cathodic potential range ( figure 3 ) and 
 more than two times larger than that of particle 1 . after this initial 
 shift , eres settled and shows less than 
 1 mev shift with increasing potential . following the reversal of the 
 potential sweep direction at 400 mv , almost no change in the resonance 
 peak position occurred until a critical potential was met , below which eres returned to its initial value ( deres / du = 24.8 mev / v ) . most importantly , the nonlinearity and hysteresis were segmented in 
 potential , indicating that the change points for deres / du have differing electrochemical 
 potentials depending on scan direction . this behavior indicates that 
 the mechanism responsible for these more drastic resonance shifts 
 is likely chemical in nature , and the resonance shift change points 
 correspond to single - particle onset potentials . in addition to potential - dependent 
 hysteresis and tuning rates for eres , 
  shows similar trends in figure 4h . 
 from 0 to 250 mv , nearly no change in plasmon resonance width occurs 
 ( d/du = 2 mev / v ) . from 250 to 400 mv , the plasmon resonance remained broadened until roughly 200 
 mv on the reverse potential scan , at which point  decreases 
 with a slope close to its initial value ( d/du = 14.4 mev / v ) . the consistent 
 bimodal deres / du and 
 d/du behavior shown in figures 4 g and 4h indicates that at 
 least one additional process along with charge depletion is occurring . non - faradaic charge density tuning on this nanoparticle resulted in 
 relatively small resonance shifts and was overshadowed by the additional 
 process that , we propose , was chemical in nature rather than physical . 
 in a single - particle steady - state experiment involving nanorods in 
 100 mm nacl on ito , dondapati et al . also observed larger shifts in 
 the plasmon resonance at small positive potentials compared to expected 
 shifts based on the mie they attributed this effect 
 to adsorbate damping due to hydration of the gold surface and excluded 
 reactions between gold and chloride ions from their treatment . in their experiment , they covered a very large 
 potential range ( 1 to + 2 v , no reference electrode ) , preventing 
 them from observing the type of hysteresis seen here in the more limited 
 potential range . unfortunately , their use of a two - electrode electrochemical 
 cell precludes the direct comparison between reported applied potentials 
 and other literature values or the present study due to unknown interfacial 
 potential differences and considerable ir drop across 
 the low concentration electrolyte . in 
 a series of dynamic ensemble spectroelectrochemical studies of many 
 gold nanoparticles on ito support in nacl , sannomiya and dahlin et 
 al . investigated the formation of a gold chloride surface complex . by measuring the ensemble absorption 
 spectrum of thousands of nanoparticles in parallel , they concluded , by observing strong scan 
 rate dependence even below + 600 mv , that metal - halide formation dominated 
 the plasmonic response almost entirely . because the ensemble absorption 
 spectra were comprised of the collective absorption of many thousands 
 of nanoparticles , it is unreasonable to expect a clearly defined onset 
 potential for this chloridation process based on our own observations . 
 in considering these explanations , we note that both tuning models 
 attribute resonance shifts at small anodic potentials to a process 
 other than charge density tuning . however , the adsorbate damping model 
 attributes resonance shifts and spectral broadening to a physical 
 effect , whereas metal halide formation obviously is a chemical process 
 that then results in damping due to a layer of gold chloride . for 
 comparison , we performed an otherwise identical experiment using a 
 more inert electrolyte , aqueous sodium phosphate . in the cathodic 
 and anodic potential ranges , we observed only linear changes in eres and . experiment details and sample 
 control experiment data are shown in section 2.3 of the supporting information . the effect of refractive 
 index modulation of the ito substrate as a result of electrochemically 
 gated charge carrier concentration changes was considered through 
 simulations . the drude  lorentz model was used to find the carrier 
 concentration - dependent refractive index of ito . this result was then 
 used for the embedding dielectric medium in mie scattering simulations 
 of a 50 nm au sphere . even in the limiting case of a sphere completely 
 embedded in ito , we found that the measured resonance energy shifts 
 would require charge carrier concentration modulation over 6 orders 
 of magnitude larger than those reported in the relevant electrochemical 
 ito gating literature . we concluded that 
 potential - dependent refractive index changes of the substrate are 
 insignificant in our system . additional details and calculation results 
 are shown in section 4 of the supporting information . our experiments showed that even at limited positive potentials 
 some nanoparticles showed clear onset potential behavior , while others 
 showed only linear plasmon shifts consistent with the charge density 
 tuning model . although we can not exclude interface damping as a tuning 
 mechanism , the heterogeneous onset potentials shown in some nanoparticles 
 ( e.g. , particle 2 ) suggest that at least one mechanism is chemical 
 in nature , possibly metal - halide formation . in terms of this working model , we postulate that the onset potential 
 for particle 2 was within the potential range , while the onset potential 
 for particle 1 was likely above the upper potential vertex ( + 400 mv ) . we conclude that even within a very narrow subset of nanoparticles 
 chosen based on behavior in the cathodic potential range further heterogeneity 
 is apparent in the limited anodic potential range investigated . in 
 light of these results , disagreement between previous single - particle 
 and ensemble spectroelectrochemical measurements however , our results show that electrochemically induced 
 changes to nanoparticle plasmons and the associated physical and chemical 
 mechanisms differ greatly within populations of nanoparticles . this study shows that spectroelectrochemical tuning of the plasmon 
 resonance is the result of multiple mechanisms and that the presence 
 and distribution of mechanisms varies from nanoparticle to nanoparticle . 
 under steady - state cathodic potentials , the majority of nanoparticles 
 demonstrated behavior inexplicable within the charge density plasmon 
 resonance tuning model . these nanoparticles were investigated under dynamic 
 potential control , and their plasmonic response to an applied electrochemical 
 potential was found to be completely reversible . for positive potentials , 
 some of these nanoparticles showed signs of an additional tuning mechanism , 
 likely chemical rather than physical in nature . this result suggests 
 that some combination of nanoparticle or nanoparticle / substrate properties 
 of these nanoelectrodes dictates their charge transfer or charge storage 
 capability . the methodology and techniques developed herein allow 
 the quantitative study of catalytic and electrochemical activity of 
 single nanoparticles , both statistically and dynamically . we hope 
 that this proof of principle study of heterogeneous single - particle 
 electrochemistry will inspire future investigations and serve as a 
 tool in informing design principles for the engineering of specific 
 electrochemical and catalytic nanoparticles and nanostructures . ongoing 
 work focuses on in - depth study of the multiple mechanisms first reported 
 here by measuring the effects of other electrolytes , ion concentrations , 
 nanoparticle morphology and size , potential ranges , and electrode 
 materials .
a hyperspectral imaging method was 
 developed that allowed the identification 
 of heterogeneous plasmon response from 50 nm diameter gold colloidal 
 particles on a conducting substrate in a transparent three - electrode 
 spectroelectrochemical cell under non - faradaic conditions . at cathodic 
 potentials , we identified three distinct behaviors from different 
 nanoparticles within the same sample : irreversible chemical reactions , 
 reversible chemical reactions , and reversible charge density tuning . 
 the irreversible reactions in particular would be difficult to discern 
 in alternate methodologies . 
 additional heterogeneity was observed 
 when single nanoparticles demonstrating reversible charge density 
 tuning in the cathodic regime were measured dynamically in anodic 
 potential ranges . 
 some nanoparticles that showed charge density tuning 
 in the cathodic range also showed signs of an additional chemical 
 tuning mechanism in the anodic range . 
 the expected changes in nanoparticle 
 free - electron density were modeled using a charge density - modified 
 drude dielectric function and mie theory , a commonly used model in 
 colloidal spectroelectrochemistry . 
 inconsistencies between experimental 
 results and predictions of this common physical model were identified 
 and highlighted . 
 the broad range of responses on even a simple sample 
 highlights the rich experimental and theoretical playgrounds that 
 hyperspectral single - particle electrochemistry opens .