Report
Recent Advances in Scanning Electrochemical Microscopy BIfc
ecause ultramicro-
the advances in SECM up to 1993 (3-7); here we wiil concentrate on later developments.
steadv-statecurrent that is relatively immune to convection thev can be used to scan the surface of a samole to imasre its
Principles of SECM
rnnopranhv and nrnhe its chemical reactivity This mnahilttv is t h e basic idea b e h i n d scanning electrochemical microsconv CSFClVn (1 ?\ S F C M i s ,. . ,, ', c*rnc\.em\ 1
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cal method but rather a drfferent way of doing electrochemistry. Almost any kind of electrochemical measurements, such as those obtained by cyclic and ac voltammetry and potentiometry, can be taken by SECMwhich adds spatial resolution that greatly increases the capacity of elecu
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neu y regress in 5>t,u as Deen rapia, ana numerous applications in eiectrocnemicai I l l C a S U l C l l l C l l t O j l l l l c L g l l l g ; CU1U l l l H . I U l a . U I l "
cation nave been reng,ted. mbCM has been nsed to probr varioud .onductive d l l U lIlouid-Llllg o u U S u aLGo, lllv.luulllg
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Michael V. Mirkin CUNY—Queess
College
0003-2700/96/0368-177A/$12.00/0 © 1996 American Chemical Society
A typical SECM experiment is performed in a three- or fourelectrode mode. A UME tip (usually a micrometer-sized disk embedded in an insulator) serves as a working electrode. The tip potential is controlled versus the reference electrode, and current is flowing between the tip and counter electrodes. The sample usually called the substrate also be biased and serve second working electrode In this four-electrode potentiostat (or bipotentiostafl is used to control the tip and snh-
SECM is a different stratf* nntentiflls Ttip> elprtrodpc pt*p im m e r s e d in a solution containing redox way of doing m e d i a t o r (e g an oxirii/able s n e c i e s electrochemistry that Red") When a sufficientl nositive noten tial is applied to the UME, the oxidation of R d t t h TTMEf rsatart brings spatial erned by the diffusion of Red to the UME resolution to the study (Figure la). When the tip is far from the substrate, the diffusion-limiting steadyof interfaces, kinetics, state tip current ^^ is given by h> - ^FD^c^a (1) and single molecules 4/ 1 •
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Analytical Chemistry News & Features, March 1, 1996 177 A
Report
where n is the number of electrons transferred per molecule of Red, F is the Faraday constant, Z)Red is the diffusion coefficient (cm2/s), cRed is the concentration (mol/cm3), and a is the disk radius. When the tip is brought close (within a few tip radii) to the substrate, the oxidized species (Ox) then diffuses to the substrate where it can be re-reduced (Figure la). This process produces an enhancement in the faradaic current at the tip electrode called "positive feedback" or iT >iT„ where iT is tip current. If the substrate is an electrical insulator and no reactions of the tip-generated species (Ox) occur at its surface at a small tipsubstrate distance d the current at the tip electrode will decrease because the insulator blocks diffusion of species Red to the tip from the bulk solution Indeed the closer the tip to the substrate the smaller
electrodes. Numerical treatment was reported for processes involving reagent adsorption on the substrate and surface diffusion (3). Besides feedback mode, several other SECM modes exist. In collection mode (Figure lb), the tip current zT is used to monitor the fluxes and concentration pro-
the current at the tin electrode or "negative fepHback" will hp T h u s hv scanning t h e tip
over the substrate surface variations of the tip current can be related to c h a n g e s in distance and hence to sub strate t h
An advantage of SECM compared with other types of scanning-probe microscopy, such as scanning tunneling or atomic force microscopies, is the availability of well-developed quantitative theory (3-7). Dimensionless current-distance curves were obtained numerically for insulating and conductive substrates (2). An analytical expression for a conductive substrate can be fit to numerical results to yield an equation that relates lr to the eistance between the conductive substrate and the tip with its radius a This fits the interval representing a distance of 0 05/2 to 20/z within 0 7% For an insulating substrate another eauation is itsf^H that is
accurate to within
0 5% over
ttie s a m e d i s t a n r p interval
Byfittingan experimental currentdistance curve to theory, the point where the tip-substrate separation equals zero Figure 1 . Three modes of SECM. can be determined, which in turn allows (a) The SECM instrument is operated in the determination of the normalized dis- feedback mode with a reversible redox mediator to probe surface reactivity, (b) In tance between the substrate and tip, escollection mode, a UME tip monitors ionic sential to quantitative SECM measurefluxes of species generated at the substrate. ments. Analytical approximations are also (c) In penetration mode, structure, kinetics, and transport processes can be studied by available for slow heterogeneous kinetelectrochemical measurements taken at the ics, at both the tip and substrate electip as it penetrates an electroactive film. trodes, and for homogeneous chemical reactions the solution gap between 178 A inAnalytical Chemistry News two & Features, March 1, 1996
files of species generated or consumed at the substrate. If the substrate is an electrode, ij-Zii (where is is substrate eurrent) represents the collection efficiency in the same manner as it is defined for the rotating ring-disk electrode. The rate of the heterogeneous chemical reaction occurring at the substrate (e.g., crystal dissolution) can also be evaluated from the tip current. In penetration mode (Figure lc), a small tip is used to penetrate a microstructure (e.g., a submicrometer-thick polymer film [8, 9]) and extract spatially resolved information about concentrations as well as kinetic and mass transport parameters. SECM probes
The information obtainable from SECM measurements depends mostly on the type and size of the scanning microprobe. Early experiments used micrometer-sized metal or carbon disks, which are still the most popular amperometric tip electrodes. However, many important species are difficult to oxidize or reduce, and thus they cannot be monitored amperometrically. For some of these species, such as CI", NH4, and alkali metal cations, a potentiometric ion-selective microelectrode (ISME) be made and used as a tip. A potentiometric tip is a passive sensor, that is, it does not change the concentration profile of electroactive species generated (or consumed) at the substrate. Thus, potentiometric tips are convenient for collection-mode measurements, such as monitoring the CI" flux at a polyaniline-coated electrode using an Ag/AgCl tip (10). Using an antimony-based pH sensing tip, one can map the pH changes accompanying electrochemical, corrosion, and biocatalytic rtrocesses and reactions and metabolic TTTY1 (*(* S S f1 S
of microorganisms (11). An antimony tip work in amperometric and potentiometric regimes After using the am-
perometric function of the tip to approach the surface and calibrate the distance 00tentiometric measurement'; can be perffirmpH Hnrrnck
