J.L#.p_,ls Published on Web 07128/2009 Electrochemistry of Undoped Diamond Nanoparticles: Accessing Surface Redox States Katherine B. Holt,* Daren J. Caruana, and Enrique J. Millán-Barriosr Departnent of Chemisrty Uniuersitl, ColLege Lontlon,20 Gordon St,London, WCIH 1A.t Received March 20,2009; E-mait: k.b.holt@ucl.ac.uk We report the electrochemical response of undoped, insulating diarnondnanoparticles. which ,uve attribufe to the oxidation and reduction of surface states. 'I'he potentials of the surface states are pH-dependent, and tiey are able to interact with soluti<¡n redox species. such as Fe(CN)or /a . 5 nnl detonation nanodianrond (ND) particles are undoped:the resistance of the dry porvder is very high, and wlren pelletedto lirrnt an electrode no electrochelnical response is obtaineddher than that of a resistor.However, there ¿uereports ol' its incorporation inttr elect¡odes for the detecrion of nirriter i¡nd into glucose biosensors,z where ND was lilund to enhance the oxygen reduction current. Recentlythe use of ND for the di¡ect electrochernical detectionof hernoglobin wasreported.-3 We have sh<:lwn that electr<xlc-irnnlobilized ND enhanced the currentresponse for different redox species.'r Here we showthe conditions ¡¡:quired fi)r electron Lransfer 1ET) between a solution rcdox couple andND and provide evidence for the existence of suface smtes at specitic pH,dependent p<.rtentials. ND particles o1'average diarneter 5 nnl havea coreol-crystalline spl carbon. with the outer 3-4 atomic layels exhibitingincr.casing arnorphous character.s Ranran spectra show no extellded graphitic content;any spr carbon present is localized and associared with specific surf ace detbcts.'rt'FTIR spectrosoopy shows the ND surluce to be ternlinated wilh oxygencontaining nloieties. rnainlyin higher. oxidation states (e .g., (l:O).4 7 J'heelec[ochernistry ol'the ND, drop-coated onto a bolon-doped diarnond (BDD) electrode, wasinvc-stigated using cyclic voltantmetry (CV) anddif'l'ercnrial pulse voltarnnrerry (DPV). ND powder. obrainecl commercially, rvas furthel purified by treatrnent at 425 'C in air.? 0.5 * 0.05mg ol'ND powder rvas suspended in I .5 * 0.2 mt- ol'erhanol. and a l-50 ¡rL aliquot of the suspension was drop-coated onto the BDD elecüode to form a ¡nror:s layer of ND of approxintately .tr -200 monolayers(see Supporting lnfornlation (SI))_ Experinlents were canied out in 0.2 M phosphate br.rll'er solulions (PBS)at pH,5-9 and with Fe(CN)63 A or LCl6r /r' present as a redox ¡:robe. The CV rcsponse fi¡r the reducrion o1' I 00 ¡rM FeICN)¡,r (pH 7) at an ND¡nodified electlode is sinlilal to rhat at bareBDD, rvith only a small enhanceruent t>bserved in the reduction current (Figure la). However. rvhenthe concentration of Fe(CN)¿,3-- is lowered to < l0 pN{, a significantenhancement in current is observedfbr the ND- moditred electrocie, asshownin Figure lb (for I pM Fe(CN)rJ-). Such enhancement.s can only be observed at scanr¿ltes <100 mV s-l; at higher scanraLes a decrea.se in reductioncurent is observed (F-igure lc). suggesting thattheND is acting as an inert, blu;king layerunder these conditions. At slow scanrates,increasing the coverage of ND fiom 4 monolayers (Figure lb) to 40 rnonolayers (Figur.e lcl) results in a 3-fold increase in reduction cu[ent. The S-shaped current response is asyrnmetricrvith reduction cunents being considerably rnore enhanced than the oxidation currents. _, r Fac'ul¡¿d de Ciencias,Departamento dr. Quirnica. Grupo cle Electfoquírnica Universidadde los Andes. \4érida,5101. Venezuela. 11272t J. AM. CHEM. SOC. 2009, 131.11272-1127s -36- Figure 1. CVs of'Fe(CN)¡r reduction at a ND-modilierl BDI) electrode (red) in 0.2M pll 7 PBS (a)4 nronolayer (l\41.) ND. J00¡M Iie(CN)6r , l0 nrVs r: (b) 4 N,fL ND, I ¡rM Fe(CN)6r , l0 rnVs r: (c)4 ML ND, I ¡M Fr(CN)nr .5(i0 mV s ¡; (d).1(l IUt- ND, I ¿N4 Fe(CN)¡r , l0 rnVs r. Illacklincs: CVs Lrndcr sanrc conclitions at a trarc BDD clcctrodc. The oxidation of I 4M Fe(CN)o't ar ND-modil'ied electrodes wasinvestigated in dill'erent pH solutions (Figure 2a -e.). The shape of the CV variessignilicantlywith pH. but enhanced cuffents are alwaysobserved in the pl'esence of ND. For solutions of pH < 7 a ret'luction currentof 7- l0 nA florvsat the startof the CV scanat 0 V, although no rr-ducible species were intentirxally prcscnt in the solution.Du|ing the CV, enhanced reduction currents are observcd al pH 7 ancl below, but a lnore syrnmetrical response is obtaineti at pH 8, and at pH 9 a 2x inclease in oxidarion current is observed above 0.3 V, rvith liltle cnhancenlc'nt in recluctiorr. The CV responsL' of an Nl)-nrodifie"d electrode (4 rnonolayer) in background PBS (withour Fe(CN)04 ) rvrs identicalto the b¿re BDD elect¡ocle in this potential range. Horvever, theelect¡.ochernical response of a thicker layer of ND (--200 monolayer) could be obsc'rved (Figure 2l-j) using l)PV. The f'eatureless re-sponse of the underlying BDD electrode has beensubtracred fbr clarity (see SI) so the obse'rved peaks can be unarnbiguously assigned to redox processes of the ND layer.I'he elecrrochemisrry of the ND is pH- depenclent, and in all cascs the oxidalionresponse is nrore conrplex thanthereduction. Sorne processes areeleclrochernically reversible. as they exhibit l'eduction and oxidationpeaks centered at a similar potential, for example, the reversible couple marked (") which shifts -(r0 rnV per pH unit. The oxiclation of ND at pH 9 is dominatetl by large, featureless fesponses above 0.35 V rvithno corresponding reduction currents. This oxidationevent is also pH-depenclent and shifls to higherpotentials as pH is decreased. I'he DPV responses inclease in cornplexityas pH is decreased. At pH 5 fr¡ur distinct oxidation petrks below 0.5 V are present. with corresponding reduction peaks obtained on the reverse scan. Redox activity associated with bulk properties shouldbe snrface inseusitive and therefbre independent o1'pH,indicating that the observed refipouse of the ND must take place via su¡lace stares. The broad peaks oblainedare likely due to surfacefirnctionalgroups r.rndergoing o;iidation or reduction at specific pH-dependent potentials. Possible 400 ^, ot -- , /o _"r::1,.r_/ ",b) ' -* ,"-.i E:t'--- t .-.'' / / .', 10.1021/ia902216n CCC: $40.75 !" 2009 American Chem¡@t Soc¡etv