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Electrochemical STM of condensed guanine on graphite
293
J. Electroanal. Chem., 312 (1991) 293-300 Elsevier Sequoia S.A., Lausanne
JEC 01708 Preliminary note
Electrochemical
STM of condensed guanine on graphite
R. Srinivasan *, J.C. Murphy and R. Fainchtein Applied Physics Laboratory
Johns Hopkins University, Laurel, MD 20723 (USA)
N. Pattabiraman ’ Code-6030, Naval Research L.aboratory, Washington, DC 20375 (USA) (Received 29 May 1991; in revised form 2 July 1991)
INTRODUCTION
Several organic molecules undergo phase transition from normal adsorbed state to condensed state at mercury/ electrolyte interface. Electrochemical evidence suggesting such a transition was first obtained by Lorenz [l] and later by Frumkin and Damaskin, who also suggested the possibility of two-dimensional condensation [2]. Quantitative analyses of the condensation process were first reported by Sathyanarayana for camphor on mercury through double layer capacitance measurements [3]. Condensation of guanine, several other purines and pyrimidines [4] and their derivatives [5] on mercury was first observed by Vetterl. Several studies have since been reported on the kinetics of nucleation and growth of these phase transitions. Two recent review articles, one by de Levie [6] and the other by Buess-Herman [7] provide a comprehensive overview of these types of organic deposition on mercury. These condensed layers have several interesting properties [6-S]: (1) the transition from a normal adsorbed state to the condensed state can be controlled by the electrode potential, the temperature or the concentration of the adsorbate in the bulk electrolyte; (2) the transition can be reversed upon variation of the parameters m (1) characteristic of a phase transition; (3) in several cases, condensation can occur without the organic molecule being electrochemically reduced or oxidized; (4) the ionic species present in the supporting electrolyte play an important role in the formation of condensed layers; (5) ions and electrons can be
l
To whom correspondence should be addressed.
’ Permanent address: Geo-Centers Inc., 10903 Indian Head Highway, Fort Washington, MD 20774, USA. 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia S.A.
294
transferred through these condensed layers with the kinetics of the transfer related to the surface pressure. Today, more than three decades after Lorenz’s findings [l], two important questions have remained unanswered: (1) can the condensation process occur on a solid electrode? (2) can the presence of the condensed state be verified by direct observation? It is possible to answer these questions by applying near field techniques such as scanning tunnelling microscopy (STM) or atomic force microscopy (AFM) on solid electrodes. In this paper we report STM images of guanine adsorbed and condensed at different electrode potential values on a highly oriented pyrolytic graphite (HOPG) electrode. Interfacial capacitance measurements have been used to establish the electrochemical conditions for adsorption and condensed layer formation. Condensation of guanine on HOPG is also shown to exhibit some of the phase transition properties described above. EXPERIMENTAL
A highly oriented pyrolytic graphite of ZYA grade from Union Carbide Coatings and Service (geometric surface area = 0.266 cm’) was used as the working electrode. A silver/silver chloride electrode without liquid junction formed the reference. A platinum wire was used as the counter electrode. A platinum-iridium (20%) wire which was etched in 20% potassium cyanide and coated with nail polish served as the tunnelling tip. The background electrolyte was 0.1 M sodium chloride, prepared by dissolving A.C.S. reagent NaCl from Aldrich in “ultrapure” water from Alfa. The electrolyte was treated with activated charcoal for a few hours and filtered through a sintered glass. A saturated solution of guanine (0.11 mM) in 0.1 M NaCl was prepared from Sigma Grade guanine as obtained from Sigma. Solutions were deaerated with argon and then added to the electro~he~cal cell. The HOPG surface was freshly cleaved before each experiment. The electrolyte was added within ten minutes after the cleaving. The tunnelling experiments were completed within ten minutes after the addition of the electrolyte. All experiments were conducted at room temperature (21 of 2” C) in an open cell covered with an aluminum bell jar, which also acted as a faraday cage. Tunnelling in the presence of the electrolyte was always done under electrochemical potential control using a bi-potentiostat contained in a Nanoscope II (Digital Instruments) STM instrument in the electrochemical (ECSTM) mode. The tunnelling current was set to 1 nA. The faradaic current through the tip was less than 0.1 nA (tip vs reference potential was between 250 and 305 mV) during the scan. The images were taken in the constant height mode at a scan rate of 31 Hz. A reference image of the graphite surface in the absence of electrolyte was taken in the air STM mode. The Nanoscope II was also used to measure the dc current-potential characteristics of the electrochemical processes for both the graphite and the Pt-Ir tip. Double layer capacitance measurements of the graphite/electrolyte interface were made using a 1 kHz, 5 mV p-p sine wave in a separate experimental setup similar to one described in the literature [9],
295 RESULTS AND DISCUSSION
A cyclic voltamogram of the graphite electrode in an electrolyte containing 0.11 mM guanine and 0.1 M NaCl showed that guanine is electrochemically stable between -0.2 and +0.7 V. This is in conformity with an earlier observation [lo]. The double layer capacitance values for the graphite electrode obtained using ac voltametry are shown in Figs. 1 and 2. The arrows in the figures represent the direction of the potential scan. The lower values of the capacitance in presence of guanine (curve (b) in Fig. 1) suggest adsorption of the organic at potentials positive of -0.15 V. During the forward potential scan from -0.2 to + 0.7 V, at the rate of 10 mV/s, a relatively sharp transition to a lower capacitance value may be seen near +0.8 V. The lowered capacitance region is generally known as a capacitance “pit” [6,7]. This is indicative of the formation of a well-organized phase of the adsorbate at potentials positive of +O.OS V compared to the normally adsorbed state between i-0.08 and -0.15 V. During the reverse potential scan, a transition from the lower to the higher capacitance value occurs at about -0.1 V. The hysteresis associated with the transition is also clearly seen in the range + 0.08 to -0.1 V. Often these transitions could not be observed if the HOPG surface was aged, but, cleaving the surface almost always revived these processes *. The transition potential during the forward scan had a monotonic dependence on the scan rate (Fig. 2), suggesting a nucleation and growth process. These properties are similar to those observed for guanine and several other dissolved organic molecules at the mercury/electrolyte interface, where the electrochemical evidence supports strongly the postulate of a phase transition to condensed layer formation [6,7]. Although no electrochemical observations on any other solid metal/electrolyte interface have been published, a capacitance “pit” for thymine on the cadmium (0001) plane has recently been observed by Vitanov, et al. [ll]. Direct evidence for condensed layer formation at the graphite interface under electrochemical potential control was obtained by STM. The results are shown in Figs. 3-5. These images were lowpass filtered to remove spatial frequencies below 0.15 nm. Figures, 3a, 4 and 5 are perspective views of the STM images of the electrode/electrolyte interface. Figure 3b is the top view of Fig. 3a. Figure 3c shows a height profile along one of the vertical column in Fig. 3b. Figures 3 and 4 show well-organized structures over 10 x 10 nm area of the interface at +0.202 and +0.505 V, respectively. Images taken over areas up to 50 X 50 nm of the interface showed that the surface structure was periodic and consistent with those in Figs. 3 and 4. Further increases in the area of the imaged surface gave little or no additional information. The electrode potentials of f 0.202 and + 0.505 V are within the range
“g of HOPG resulted almost always in a surface with steps. However, as shown by STM images taken in air, several islands of atomically smooth surface larger than 100 x 100 nm were easily obtained. The inability to obtain atomic flatness over the entire geometric surface did pose some problems in the double layer measurements. The absolute capacitance values were not reproducible between different cleaved surfaces.
296
0.6
0.4
0.2 Wks
0.0
-0.2
AglAgCI )
Fig. 1. Double layer capacitance of graphite in (a) 0.1 M NaCl and (b) 0.1 M NaCl saturated with guanine. The steps in the capacitance values represent phase transitions at the interface. The arrows in Figs. 1 and 2 represent the direction of the potential scan.
where a stable condensed phase is suggested by the capacitance measurements shown in Fig. 1. The differences seen between Figs. 3 and 4 may be one of perspective or position of the tunnelling tip relative to the substrate. When the potential was changed to - 0.189 V, the STM image (Fig. 5, which was enlarged x 2 from a original of 10 X 10 nm) resembled that of the graphite surface taken in air. This indicates the absence of any organized organic layer at the interface. Note that at -0.189 V the capacitance curve in Fig. 1 also suggests the absence of the condensed layer. When the potential was changed back to any value in the 0.2 to 0.6 V range, well-structured condensed layer images were obtained. No clear image could be obtained at potentials positive of 0.6 V. It is also worthwhile noting that the STM images of the graphite surface taken using pure 0.1 M NaCl with no guanine in the electrolyte resembled Fig. 5 closely over the entire range of the electrode potential (- 0.2 to + 0.7 V). CONCLUSIONS
The STM images can be used at a qualitative level to characterize the arrangement of guanine in the condensed state. The image in Fig. 3 shows an array of peaks
ENvs
AS/AgCI)
Fig. 2. Double layer capacitance vs. electrode potential for graphite in 0.1 M NaCl saturated with guanine measured at scan rates of (a) 10 mV/s; (b) 20 mV/s; (c) 50 mV/s; (d) 100 mV/s. The scan rate dependence for the ,transition potential indicates nucleation and growth process.
297
Fig. 3. STM image of the condensed guanine layer on graphite at +0.202 V obtained with a tip-sample bias potential of + 60 mV. (a) Perspective view; the insert is a stick model of the guanine molecule drawn to scale (Y-axis) of the STM image; (b) top view; the inset is a stick model of the guanine mokxule drawn to scale; (c) height profile along one of the vertical cohmm in (b). These images, as well as those in Figs. 4 and 5, were filtered in order to remove spatial frequencies below 0.15 nm.
298
Fig. 4. STM image of the condensed bias potential of - 193 mV.
guanine layer on graphite at +0.505
V obtained with a tip-sample
comprising about six near-vertical columns and 16 diagonal rows. A stick model of the guanine molecule drawn to scale and included as an inset in Figs. 3a and 3b helps compare these dimensions to some of the features in the STM image. Assuming a flat orientation for the condensed guanine molecules at the HOPG interface, the area projected by the stick model in the plane of the molecule is comparable to the area of each peak in Fig. 3. Furthermore, in the area between two vertical columns two planar guanine molecules will fit, provided the distance of separation between them is comparable to hydrogen bond length. Also note that the repeat distance between the rows is approximately 0.9 nm and between the columns about 1.7 nm. Similar ordering of the molecules may be identified in solid guanine, for example, in its monohydrate crystal in the lattice structure projected on the (301) plane [12], see Fig. 6 In such a lattice each guanine molecule has six hydrogen bonds (accounting for the stability of the structure) and is oriented almost parallel to the projected plane. However, detailed analysis of the observed structure of the condensed layers require study of several properties of the interface, including the role of epitaxy on the graphite electrode and surface reconstruction in presence of the electrolyte. The observations reported in this paper demonstrate the formation of condensed layers of guanine on the graphite/electrolyte interface under electrochemical potential control. It also provides non-electrochemical evidence for the change in the state
299
Fig. 5. STM image of the graphite/electrolyte taken in air.
interface at + 0.189 V. This resembles graphite STM image
of adsorption under the change of the electrode potential. This appears to be the first reported observation of electrochemical condensation on graphite. The ability to obtain STM images of the condensed guanine layer at the level of molecular
t 2.0 nm i
Fig. 6. A diagram of the lattice structure of guanine m~n~hydrate crystal projected in the @Ol) plane, adapted from ref. 12.
300
resolution may help understand several properties of the layer. Extension of the STM or AFM-Electrochemical methods to other condensing organic systems on solid electrodes other than graphite should be possible. ACKNOWLEDGEMENTS
The authors wish to thank Professor Robert de Levie, Georgetown University, for bringing to our attention the thymine condensation on cadmium (ref. 11) and for his critical comments. Financial assistance by the Bio-Medical Program Office at the Johns Hopkins University, Applied Physics Laboratory is acknowledged. REFERENCES 1 W. Lorenz, 2. Elektrochem., 62 (1958) 192. 2 A.N. Frumkin and B.B. Damaskin, Dokl. Akad. Nauk SSSR, 129 (1959) 862; A.N. Frumkin and B.B. Damaskin, in J.O’M. Bockris and B.E. Conway (Eds.), Modem Aspects of Electrochemistry, Vol. 3, Butterworths, London, 1964, Ch. 3. 3 S. Sathyanarayana, J. Electroanal. Chem., 10 (1965) 56; K.G. Baikerikar and S. Sathyanarayana, J. Electroanal. Chem., 21 (1969) 449. 4 V. Vetterl, Experientia, 21 (1965) 9; V. Vetterl, Collect. Czech. Chem. Commun., 31 (1966) 2105. 5 V. Vetterl, J. Electroanal. Chem., 19 (1968) 169. 6 R. de Levie, Chem. Rev., 88 (1988) 599. 7 Cl. Buess-Herman, J. Electroanal. Chem., 186 (1985) 27. 8 R. Srinivasan and R. de Levie, J. Electroanal. Chem., 201 (1986) 145. 9 R. de Levie and A.A. Husovsky, J. Electroanal. Chem., 20 (1969) 181. 10 R.N. Goyal and G. Dryhurst, J. Electroanal. Chem., 135 (1982) 75. 11 T. Vitanov et al., private communication via R. de Levie, 1991. 12 U. Thewalt, C.E. Bug, R.E. Marsh, Acta Cryst., B27 (1971) 2358.
J. Electroanal. Chem., 312 (1991) 293-300 Elsevier Sequoia S.A., Lausanne
JEC 01708 Preliminary note
Electrochemical
STM of condensed guanine on graphite
R. Srinivasan *, J.C. Murphy and R. Fainchtein Applied Physics Laboratory
Johns Hopkins University, Laurel, MD 20723 (USA)
N. Pattabiraman ’ Code-6030, Naval Research L.aboratory, Washington, DC 20375 (USA) (Received 29 May 1991; in revised form 2 July 1991)
INTRODUCTION
Several organic molecules undergo phase transition from normal adsorbed state to condensed state at mercury/ electrolyte interface. Electrochemical evidence suggesting such a transition was first obtained by Lorenz [l] and later by Frumkin and Damaskin, who also suggested the possibility of two-dimensional condensation [2]. Quantitative analyses of the condensation process were first reported by Sathyanarayana for camphor on mercury through double layer capacitance measurements [3]. Condensation of guanine, several other purines and pyrimidines [4] and their derivatives [5] on mercury was first observed by Vetterl. Several studies have since been reported on the kinetics of nucleation and growth of these phase transitions. Two recent review articles, one by de Levie [6] and the other by Buess-Herman [7] provide a comprehensive overview of these types of organic deposition on mercury. These condensed layers have several interesting properties [6-S]: (1) the transition from a normal adsorbed state to the condensed state can be controlled by the electrode potential, the temperature or the concentration of the adsorbate in the bulk electrolyte; (2) the transition can be reversed upon variation of the parameters m (1) characteristic of a phase transition; (3) in several cases, condensation can occur without the organic molecule being electrochemically reduced or oxidized; (4) the ionic species present in the supporting electrolyte play an important role in the formation of condensed layers; (5) ions and electrons can be
l
To whom correspondence should be addressed.
’ Permanent address: Geo-Centers Inc., 10903 Indian Head Highway, Fort Washington, MD 20774, USA. 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia S.A.
294
transferred through these condensed layers with the kinetics of the transfer related to the surface pressure. Today, more than three decades after Lorenz’s findings [l], two important questions have remained unanswered: (1) can the condensation process occur on a solid electrode? (2) can the presence of the condensed state be verified by direct observation? It is possible to answer these questions by applying near field techniques such as scanning tunnelling microscopy (STM) or atomic force microscopy (AFM) on solid electrodes. In this paper we report STM images of guanine adsorbed and condensed at different electrode potential values on a highly oriented pyrolytic graphite (HOPG) electrode. Interfacial capacitance measurements have been used to establish the electrochemical conditions for adsorption and condensed layer formation. Condensation of guanine on HOPG is also shown to exhibit some of the phase transition properties described above. EXPERIMENTAL
A highly oriented pyrolytic graphite of ZYA grade from Union Carbide Coatings and Service (geometric surface area = 0.266 cm’) was used as the working electrode. A silver/silver chloride electrode without liquid junction formed the reference. A platinum wire was used as the counter electrode. A platinum-iridium (20%) wire which was etched in 20% potassium cyanide and coated with nail polish served as the tunnelling tip. The background electrolyte was 0.1 M sodium chloride, prepared by dissolving A.C.S. reagent NaCl from Aldrich in “ultrapure” water from Alfa. The electrolyte was treated with activated charcoal for a few hours and filtered through a sintered glass. A saturated solution of guanine (0.11 mM) in 0.1 M NaCl was prepared from Sigma Grade guanine as obtained from Sigma. Solutions were deaerated with argon and then added to the electro~he~cal cell. The HOPG surface was freshly cleaved before each experiment. The electrolyte was added within ten minutes after the cleaving. The tunnelling experiments were completed within ten minutes after the addition of the electrolyte. All experiments were conducted at room temperature (21 of 2” C) in an open cell covered with an aluminum bell jar, which also acted as a faraday cage. Tunnelling in the presence of the electrolyte was always done under electrochemical potential control using a bi-potentiostat contained in a Nanoscope II (Digital Instruments) STM instrument in the electrochemical (ECSTM) mode. The tunnelling current was set to 1 nA. The faradaic current through the tip was less than 0.1 nA (tip vs reference potential was between 250 and 305 mV) during the scan. The images were taken in the constant height mode at a scan rate of 31 Hz. A reference image of the graphite surface in the absence of electrolyte was taken in the air STM mode. The Nanoscope II was also used to measure the dc current-potential characteristics of the electrochemical processes for both the graphite and the Pt-Ir tip. Double layer capacitance measurements of the graphite/electrolyte interface were made using a 1 kHz, 5 mV p-p sine wave in a separate experimental setup similar to one described in the literature [9],
295 RESULTS AND DISCUSSION
A cyclic voltamogram of the graphite electrode in an electrolyte containing 0.11 mM guanine and 0.1 M NaCl showed that guanine is electrochemically stable between -0.2 and +0.7 V. This is in conformity with an earlier observation [lo]. The double layer capacitance values for the graphite electrode obtained using ac voltametry are shown in Figs. 1 and 2. The arrows in the figures represent the direction of the potential scan. The lower values of the capacitance in presence of guanine (curve (b) in Fig. 1) suggest adsorption of the organic at potentials positive of -0.15 V. During the forward potential scan from -0.2 to + 0.7 V, at the rate of 10 mV/s, a relatively sharp transition to a lower capacitance value may be seen near +0.8 V. The lowered capacitance region is generally known as a capacitance “pit” [6,7]. This is indicative of the formation of a well-organized phase of the adsorbate at potentials positive of +O.OS V compared to the normally adsorbed state between i-0.08 and -0.15 V. During the reverse potential scan, a transition from the lower to the higher capacitance value occurs at about -0.1 V. The hysteresis associated with the transition is also clearly seen in the range + 0.08 to -0.1 V. Often these transitions could not be observed if the HOPG surface was aged, but, cleaving the surface almost always revived these processes *. The transition potential during the forward scan had a monotonic dependence on the scan rate (Fig. 2), suggesting a nucleation and growth process. These properties are similar to those observed for guanine and several other dissolved organic molecules at the mercury/electrolyte interface, where the electrochemical evidence supports strongly the postulate of a phase transition to condensed layer formation [6,7]. Although no electrochemical observations on any other solid metal/electrolyte interface have been published, a capacitance “pit” for thymine on the cadmium (0001) plane has recently been observed by Vitanov, et al. [ll]. Direct evidence for condensed layer formation at the graphite interface under electrochemical potential control was obtained by STM. The results are shown in Figs. 3-5. These images were lowpass filtered to remove spatial frequencies below 0.15 nm. Figures, 3a, 4 and 5 are perspective views of the STM images of the electrode/electrolyte interface. Figure 3b is the top view of Fig. 3a. Figure 3c shows a height profile along one of the vertical column in Fig. 3b. Figures 3 and 4 show well-organized structures over 10 x 10 nm area of the interface at +0.202 and +0.505 V, respectively. Images taken over areas up to 50 X 50 nm of the interface showed that the surface structure was periodic and consistent with those in Figs. 3 and 4. Further increases in the area of the imaged surface gave little or no additional information. The electrode potentials of f 0.202 and + 0.505 V are within the range
“g of HOPG resulted almost always in a surface with steps. However, as shown by STM images taken in air, several islands of atomically smooth surface larger than 100 x 100 nm were easily obtained. The inability to obtain atomic flatness over the entire geometric surface did pose some problems in the double layer measurements. The absolute capacitance values were not reproducible between different cleaved surfaces.
296
0.6
0.4
0.2 Wks
0.0
-0.2
AglAgCI )
Fig. 1. Double layer capacitance of graphite in (a) 0.1 M NaCl and (b) 0.1 M NaCl saturated with guanine. The steps in the capacitance values represent phase transitions at the interface. The arrows in Figs. 1 and 2 represent the direction of the potential scan.
where a stable condensed phase is suggested by the capacitance measurements shown in Fig. 1. The differences seen between Figs. 3 and 4 may be one of perspective or position of the tunnelling tip relative to the substrate. When the potential was changed to - 0.189 V, the STM image (Fig. 5, which was enlarged x 2 from a original of 10 X 10 nm) resembled that of the graphite surface taken in air. This indicates the absence of any organized organic layer at the interface. Note that at -0.189 V the capacitance curve in Fig. 1 also suggests the absence of the condensed layer. When the potential was changed back to any value in the 0.2 to 0.6 V range, well-structured condensed layer images were obtained. No clear image could be obtained at potentials positive of 0.6 V. It is also worthwhile noting that the STM images of the graphite surface taken using pure 0.1 M NaCl with no guanine in the electrolyte resembled Fig. 5 closely over the entire range of the electrode potential (- 0.2 to + 0.7 V). CONCLUSIONS
The STM images can be used at a qualitative level to characterize the arrangement of guanine in the condensed state. The image in Fig. 3 shows an array of peaks
ENvs
AS/AgCI)
Fig. 2. Double layer capacitance vs. electrode potential for graphite in 0.1 M NaCl saturated with guanine measured at scan rates of (a) 10 mV/s; (b) 20 mV/s; (c) 50 mV/s; (d) 100 mV/s. The scan rate dependence for the ,transition potential indicates nucleation and growth process.
297
Fig. 3. STM image of the condensed guanine layer on graphite at +0.202 V obtained with a tip-sample bias potential of + 60 mV. (a) Perspective view; the insert is a stick model of the guanine molecule drawn to scale (Y-axis) of the STM image; (b) top view; the inset is a stick model of the guanine mokxule drawn to scale; (c) height profile along one of the vertical cohmm in (b). These images, as well as those in Figs. 4 and 5, were filtered in order to remove spatial frequencies below 0.15 nm.
298
Fig. 4. STM image of the condensed bias potential of - 193 mV.
guanine layer on graphite at +0.505
V obtained with a tip-sample
comprising about six near-vertical columns and 16 diagonal rows. A stick model of the guanine molecule drawn to scale and included as an inset in Figs. 3a and 3b helps compare these dimensions to some of the features in the STM image. Assuming a flat orientation for the condensed guanine molecules at the HOPG interface, the area projected by the stick model in the plane of the molecule is comparable to the area of each peak in Fig. 3. Furthermore, in the area between two vertical columns two planar guanine molecules will fit, provided the distance of separation between them is comparable to hydrogen bond length. Also note that the repeat distance between the rows is approximately 0.9 nm and between the columns about 1.7 nm. Similar ordering of the molecules may be identified in solid guanine, for example, in its monohydrate crystal in the lattice structure projected on the (301) plane [12], see Fig. 6 In such a lattice each guanine molecule has six hydrogen bonds (accounting for the stability of the structure) and is oriented almost parallel to the projected plane. However, detailed analysis of the observed structure of the condensed layers require study of several properties of the interface, including the role of epitaxy on the graphite electrode and surface reconstruction in presence of the electrolyte. The observations reported in this paper demonstrate the formation of condensed layers of guanine on the graphite/electrolyte interface under electrochemical potential control. It also provides non-electrochemical evidence for the change in the state
299
Fig. 5. STM image of the graphite/electrolyte taken in air.
interface at + 0.189 V. This resembles graphite STM image
of adsorption under the change of the electrode potential. This appears to be the first reported observation of electrochemical condensation on graphite. The ability to obtain STM images of the condensed guanine layer at the level of molecular
t 2.0 nm i
Fig. 6. A diagram of the lattice structure of guanine m~n~hydrate crystal projected in the @Ol) plane, adapted from ref. 12.
300
resolution may help understand several properties of the layer. Extension of the STM or AFM-Electrochemical methods to other condensing organic systems on solid electrodes other than graphite should be possible. ACKNOWLEDGEMENTS
The authors wish to thank Professor Robert de Levie, Georgetown University, for bringing to our attention the thymine condensation on cadmium (ref. 11) and for his critical comments. Financial assistance by the Bio-Medical Program Office at the Johns Hopkins University, Applied Physics Laboratory is acknowledged. REFERENCES 1 W. Lorenz, 2. Elektrochem., 62 (1958) 192. 2 A.N. Frumkin and B.B. Damaskin, Dokl. Akad. Nauk SSSR, 129 (1959) 862; A.N. Frumkin and B.B. Damaskin, in J.O’M. Bockris and B.E. Conway (Eds.), Modem Aspects of Electrochemistry, Vol. 3, Butterworths, London, 1964, Ch. 3. 3 S. Sathyanarayana, J. Electroanal. Chem., 10 (1965) 56; K.G. Baikerikar and S. Sathyanarayana, J. Electroanal. Chem., 21 (1969) 449. 4 V. Vetterl, Experientia, 21 (1965) 9; V. Vetterl, Collect. Czech. Chem. Commun., 31 (1966) 2105. 5 V. Vetterl, J. Electroanal. Chem., 19 (1968) 169. 6 R. de Levie, Chem. Rev., 88 (1988) 599. 7 Cl. Buess-Herman, J. Electroanal. Chem., 186 (1985) 27. 8 R. Srinivasan and R. de Levie, J. Electroanal. Chem., 201 (1986) 145. 9 R. de Levie and A.A. Husovsky, J. Electroanal. Chem., 20 (1969) 181. 10 R.N. Goyal and G. Dryhurst, J. Electroanal. Chem., 135 (1982) 75. 11 T. Vitanov et al., private communication via R. de Levie, 1991. 12 U. Thewalt, C.E. Bug, R.E. Marsh, Acta Cryst., B27 (1971) 2358.