Cyanide adsorption on gold electrodes: a combined surface enhanced Raman spectroscopy and density functional theory study

A combined SERS and DFT study of cyanide adsorption on a gold electrode is presented. From our analysis, the high-frequency mode at ～2100 cm^?1 is ascribed to the C–N stretching frequency at (1 0 0) and (1 1 0) sites. The lower frequency modes at ～370 and ～300 cm^?1 are ascribed to the Au–CN stretching and bending modes, respectively. The Stark tuning slopes of these modes agree well with the DFT computations. The bending mode at 300 cm^?1 has a very small Stark tuning slope of ～4 cm^?1 V^?1, experimentally, compared to ～1 cm^?1 V^?1, computationally. The Au–CN stretching frequency has a Stark tuning slope of ～15 cm^?1 V^?1, experimentally, compared to 16–22 cm^?1 V^?1, computationally. The positive Stark tuning slopes suggest that cyanide adsorbs as an anion, with a bond polarity between that of the Au–Cl and Au–Br surface bonds. The anionic character of the Au–CN bond is also confirmed by different charge analyses based on the DFT computations. The ordering in binding strength on the three different surfaces is the same as the ordering in bond ionicity. The C–N stretching frequency has two different Stark tuning slopes in the SERS experiments: ～14 cm^?1 V^?1 in the more negative potential region corresponding to cyanide adsorption, and ～35 cm^?1 V^?1 in the more positive potential region corresponding to gold dissolution. The computational result of ～8 cm^?1 V^?1 agrees well with the lower Stark tuning slope. The higher Stark tuning slope is presumably related to the formation of gold–cyanide complexes in the double layer.     

An investigation of the adsorption of pyrazine and pyridine on nickel electrodes by in situ surface-enhanced Raman spectroscopy

This paper describes a surface-enhanced Raman spectroscopic study on nickel electrodes. The enhancement factor of the roughened Ni substrate was found to be from two to three orders, depending on the surface pre-treatment. High-quality SERS spectra of pyrazine and pyridine were obtained at different potentials. The bands assigned to Ni–N_pyrazine and Ni–N_pyridine were observed around 260 cm^?1 in the low-frequency region of two sets of SERS spectra, respectively. The adsorption behaviour of pyrazine and pyridine on Ni electrode was studied and compared based on the surface selection rule. The result suggests that both pyrazine and pyridine were strongly adsorbed onto the substrates. It also implies that pyridine was adsorbed perpendicularly onto the substrate, while pyrazine adsorbed onto the substrate in a slightly tilted vertical configuration. The observation of forbidden bands of the pyrazine molecule is due to the lowering of the symmetry from D_2h to C_2V when adsorption onto the surface takes place. The electrochemical reduction of pyrazine on the Ni electrode was observed at extremely negative potentials in the presence of Cl^?.     

Surface Raman scattering studies on the adsorption of p-hydroxybenzoic acid at Au electrodes with ultraviolet excitation

The potential-dependent surface Raman spectrum of adsorbed p-hydroxybenzoic acid (PHBA) on a roughened Au electrode has been obtained with ultraviolet excitation at 325 nm. The surface Raman spectra of PHBA are sensitive to the negative shift of potential and the electrode potential at which the resonance (potential of maximum intensity) occurs varies with the vibrational mode, indicating that the PHBA molecules are chemisorbed on the Au surface. A non-radiative charge transfer mechanism could probably explain the experiment results in the present work. The probable reasons are given.     

Surface enhanced Raman scattering (SERS) of 1-hydroxybenzotriazole adsorbed on a copper electrode surface

The adsorption of 1-hydroxybenzotriazole (BTAOH) on the copper electrode was studied by surface-enhanced Raman scattering spectroscopy (SERS) as a function of electrode potential and pH value of the solution. A striking similarity between the SERS spectra of BTAOH and the respective spectra of benzotriazole (BTAH) was observed in spite of significant differences in the normal Raman spectra of these compounds. A hypothesis assuming transformation of the BTAOH molecule into BTAH at the copper surface has been verified by H/D isotopic exchange experiments. The unique sensitivity of the SERS spectrum to isotopic exchange found in acidic medium (pH 2) at considerably negative electrode potentials together with some differences between the SERS spectra of BTAOH and BTAH indicated that BTAOH is mainly adsorbed in its molecular form. However, the transformation from the adsorbed BTAOH molecule to the surface complex of BTA^? anion with Cu(I) was suggested for alkaline solutions and potentials more positive than ?0.2 V.     

Thiourea adsorption on a Pt surface as detected by electrochemical methods and surface-enhanced Raman spectroscopy

The adsorption behavior of thiourea (TU) on a roughened Pt electrode was investigated by combining electrochemical methods and surface-enhanced Raman spectroscopy. It was found that TU could be dissociated on a roughened Pt surface at the open circuit potential to form a surface sulfide. The dissociation process can be effectively suppressed if the electrode is immersed into the solution with its potential controlled at a negative value. Using these tactics, we are able to perform a surface-enhanced Raman spectroscopy (SERS) study of the adsorption behavior of TU on Pt. The SERS results reveal that TU is adsorbed through its sulfur atom in a tilted way and the orientation changes with the applied potential. The study points out the importance of controlling the electrode potential properly upon immersion of the electrode into the solution.     

Electrooxidative coupling of a toluidine blue O terminated self-assembled monolayer studied by electrochemistry and surface enhanced Raman spectroscopy

A toluidine blue O (TBO) terminated self-assembled monolayer (SAM-I) was fabricated on a gold disk electrode by nucleophilic attack of the amino group in the reduced TBO on an immobilized carbonyl moiety of terephthaloyl chloride with cyclic scanning. The TBO units in the SAM-I were observed by surface enhanced Raman spectroscopy (SERS). The average TBO surface coverage of SAM-I was (2.5±0.5)×10^?10 mol cm^?2. Meanwhile, another TBO derived SAM (SAM-II) was prepared for comparison by activating the free carboxyl group of bound terephthaloyl chloride and then reacting it with the amino group of TBO. The average TBO surface coverage of SAM-II was only (6.8±0.1)×10^?11 mol cm^?2. Both SAMs possessed good electroactivity and showed identical properties. With an electrooxidative coupling of TBO units in SAM-I, a novel electroactive polymerized TBO monolayer (Film-I) was constructed, which displayed an improved stability and obvious electrochemical blocking effect. As a comparison, a poly(TBO) film (Film-II) was obtained by the electrochemical polymerization of TBO in solution on SAM-I for remedying the defectiveness of the knitted structure of Film-I. The SERS displayed more TBO units in Film-II than in Film-I. Film-II provided a much better barrier to the redox probe of K₃[Fe(CN)₆] due to the formation of the perfect structure.     

SERS and resonance Raman studies of p-aminoazobenzene on gold and silver electrodes

Surface-enhanced Raman scattering (SERS) spectra and cyclic voltammetry were used to detect the products of the oxidation and reduction of p-aminoazobenzene (PAAB) on roughened gold and silver electrodes in acidic and neutral solutions.The results indicated that oligomerization products are formed during the oxidation of PAAB on a gold electrode. We identified these products as oligomers formed by CN coupling of the PAAB cation radicals. The same products are formed on a silver electrode at a lower potential as the result of a photoelectrochemical reaction.The measurements carried out in the negative potential range proved that PAAB undergoes irreversible reduction in acidic media and reversible reduction to hydrazobenzene in neutral media.     

The orientation of 2,2′-bipyridine adsorbed at a SERS-active Au(1 1 1) electrode surface

Surface-enhanced Raman scattering (SERS) spectra from 2,2′-bipyridine (22BPY) adsorbed on a SERS-active Au(1 1 1) electrode at several applied potentials were obtained. The SERS-active Au(1 1 1) surface was prepared following an electrochemical cleaning procedure. This procedure involves the application of continuous oxidation–reduction cycles (orcs) within a potential region where no significant surface structural changes are expected to occur. The adsorbed 22BPY may assume several conformations, including the cis- and trans-configurations. Normal Raman spectra of aqueous 22BPY at several different acidities are presented. These spectra are compared to the Raman features of both the ‘free’ 22BPY and its Zn complex in the solid state. The normal Raman experiments showed that unique spectral characteristics were distinguishable for both the cis- and trans-configurations. Based on these results, the potential-dependent orientation (conformation) of 22BPY adsorbed on a SERS-active Au(1 1 1) has been established. The SERS results suggested that, at a positively-charged surface, 22BPY adsorbs end-on, using both nitrogens (cis-configuration). Although no strong spectroscopic evidence suggesting potential-induced reorientation was found, the pyridine rings may no longer be coplanar at the negatively-charged surface. However, the molecules keep the upright position with both nitrogens pointing towards the surface even in these negative limits.     

An electrodeposition study of the nucleation and growth of silver on boron-doped diamond electrodes

A study is reported of the electrochemical nucleation of silver on boron-doped diamond from aqueous solutions of silver nitrate, using a single-pulse potentiostatic technique. Data are obtained for the nucleation rate by interpretation of the deposition voltammetry, and by inspection of in situ optical microscopic images. The distribution of nearest neighbours is determined as a function of time and also the first to fifth nearest-neighbour distributions are examined at specific times. The distribution of nucleus areas is found and a simple stochastic model of nucleation and growth developed in order to understand qualitatively the nearest neighbour and nucleus area results.     

Electrochemical surface reactions of intermediates formed in the oxidative ethanol adsorption on porous Pt and PtRu

The oxidative ethanol adsorption and electrochemical surface reactions of adsorbed intermediates were studied on electrodeposited Pt, Ru, Pt_0.92Ru_0.08 and Pt_0.85Ru_0.15 in 1 M HClO₄. A flow cell procedure was performed in order to separate surface reactions of intermediates formed in the oxidative ethanol adsorption from those reactions for ethanol in the bulk electrolyte solution. Oxidation and reduction reactions of adsorbed species were studied by potential-controlled electrodesorption spectrometry. No adsorbate formation was observed on a pure Ru electrode. In all cases, CO₂ was the sole product formed in the oxidative electrodesorption of the adsorbates. Using ¹²CH₃¹²CH₂OH and ¹²CH₃¹³CH₂OH, the oxidation reaction pathways of each C-atom to CO₂ were followed. On the other hand, methane and ethane were detected during the reductive electrodesorption of adlayer species. The onset potential for these reduction products shifts to more negative potentials as the Ru content of the alloy increases. The methane to ethane yield ratio decreases in the sequence Pt>Pt_0.92Ru_0.08>Pt_0.85Ru_0.15. A critical interpretation of the experimental data leads to a tentative yield of different adsorbed species as function of the Ru surface concentration.     

The influence of Cu adatoms on the Zn upd on polycrystalline thin gold film electrodes: a study using surface conductance measurements

The adsorption behaviour of Zn upd and irreversibly adsorbed Cu on thin gold films in a NaH₂PO₄ electrolyte was studied applying both CV and surface conductance (SC) measurements. These processes were studied separately and the upd of Zn in the presence of previously adsorbed Cu was also studied. Zn upd and ups occur with an apparent number of transferred electrons n=1. The SC measurements show that Zn upd does not conform the surface Linde rule. The n value obtained can be explained by considering that the product is not Zn(0). The SC response of Cu irreversibly adsorbed on gold shows an increase in SC as the degree of coverage increases.The voltammetric response for the upd of Zn on Cu-covered gold electrodes shows that a Cu coverage around θ_Cu～0.66 is enough to inhibit the Zn upd process on gold completely. The charge relation (Q_Cu vs. Q_Zn) shows that one Cu adatom blocks one Zn site. The SC shows that the response of Zn on Cu pre-adsorbed on gold is additive. This last behaviour can be explained only by inferring a phase segregation in two dimensions where Cu and Zn depositions occur in the form of individual islands.     

Impact of colour digital photography on pathologists' orientation of resected specimens: a prospective pilot study

Accurate orientation of surgical resection specimens is a prerequisite for the correct histopathological study of the extent of disease and for a complete clinico-pathological interpretation. This pilot study demonstrates how the pathologists understanding of specimen orientation may be enhanced by the sending of a digital image with the pathology request form.     

The influence of resin infiltration system on enamel microhardness and surface roughness: An in vitro study

Objective

To investigate the effect of a resin infiltrant on the surface microhardness and roughness of healthy enamel and, as a subsidiary aim, to compare it with a fissure sealant.

Materials and methods

Twenty freshly extracted premolars were used. Sound enamel surfaces were treated with a resin infiltrant (Icon) or fissure sealant (Seal-Rite). The average roughness (R_a, μm) of the specimens was measured with a profilometer (Surtronic 10 Tylor Hobson). Surface hardness was determined by utilizing Vicker’s surface hardness (VHN) with a Micromet II Microhardness tester. Each specimen acted as its own control. Data were analyzed with 2-way analysis of variance (ANOVA), and mean values were compared with independent t-test. All analyses were performed with the SPSS program version 16 (USA). Differences with a P-value of ⩽0.05 were considered statistically significant.

Results

Comparison of enamel surfaces before and after application of resin infiltrant revealed no significant differences in surface hardness; however, enamel surfaces treated by infiltrant showed significantly higher VHN (244.0 ± 79.8) values than those treated with fissure sealant (37.5 ± 14.2). Surface roughness did not differ before and after application of either material to sound enamel. Enamel surfaces treated with fissure sealant (5.3 ± 1.4) were significantly smoother than those treated with infiltrant (6.9 ± 2.0).

Conclusion

Within the limitations of the study, the results showed that enamels treated with the resin infiltrant showed approximately the same microhardness and surface roughness as sound enamel, indicating that this material might be suitable for the treatment of enamel subsurface lesions.     

Underpotential deposition of lead on Pt(111) in the presence of bromide: RRDPt(111)E and X-ray scattering studies1

The underpotential deposition (upd) of Pb onto Pt(111) in solution containing Br^? was examined by classical RRD_Pt(111)E electrochemical methods in combination with in situ surface X-ray scattering measurements. The results show that Pb upd onto Pt(111) in the presence of bromide is a competitive process with the total amount of Pb deposited at underpotential being only ca. 0.51±5% ML, less than the amount needed to form a close-packed monolayer, 0.63 ML (1 ML equals 1 Pb atom per Pt surface atom). Deposition of Pb adatoms is accompanied by desorption of about one-half of the bromide adsorbed on the Pt surface at higher potentials, leaving co-adsorbed bromide both in and on-top-of the adlayer. In the potential region where the ratio of co-adsorbed species is close to one (E≈0 V) there is the slow formation (over a period of ca. 2 h) of an ordered surface alloy, which has a p(2×2) unit cell. No other ordered structures of Pb are observed in the underpotential region. © 1997 Elsevier Science S.A.     

Comparison of electrosorption at activated polycrystalline and Pt(531) kinked platinum electrodes: surface voltammetry and charge displacement on potentiostatic CO adsorption

In this work we have tried to reproduce the hydrogen adsorption behaviour of electrochemically activated polycrystalline platinum electrodes by using a kinked platinum single crystal surface subjected only to flame cleaning before checking hydrogen electrosorption at its surface. The Pt(531) surface may be considered as a model for a platinum surface which has been extensively activated by cycles of oxygen adsorption-desorption. In the hard sphere model the surface unit cell for this orientation contains three sites, with four-fold, three-fold and two-fold symmetry respectively. The electrochemical behaviour of this surface, prepared from a platinum bead, was checked in 0.5 M H₂SO₄ solution by voltammetry after flame cleaning and various cooling conditions. It was compared with that of a rough platinum surface, cut from another bead in a direction chosen at random, either studied after flame cleaning or after electrochemical activation. Experiments on charge displacement by CO adsorption at constant potentials show, for both samples, that the voltammetric charge at the two limits of the hydrogen upd range is made up of two components of opposite sign. Only 0.69 and 0.77 of the voltammetric charge on Pt(531) and rough platinum respectively may be ascribed to adsorbed hydrogen; the remaining part of this charge is involved in a reversible adsorption state formed in an anodic process. These results are discussed in terms of the inadequacy of the conventional method of correcting, from double layer effects, the voltammetric charge of activated platinum electrodes in order to measure their hydrogen surface content.     

Effect of CO2 laser on pulpal temperature and surface morphology: an in vitro study.

Objectives: The objective of this study was to evaluate the potential effects on underlying dental hard tissues of a high pulse rate carbon dioxide (CO₂) laser that was designed for soft tissue surgery.

Methods: Eighteen extracted human teeth were sectioned longitudinally, cleaned, and varnished, leaving nine exposed windows on each: six on the coronal surface (enamel) and three on the root surface (cementum, dentin). The CO₂ irradiation conditions used were: wave length 10.6 μm; 1.2–2.6 J/cm² fluence per pulse; repetition rate 120–1000 Hz; 100–200 ms pulse duration; and cumulative fluences ranging from 14 to 2200 J/cm². Each window was irradiated with a 0.3 mm beam diameter at one of nine power settings for 0.1, 0.5, or 1.0 s. The pulp chamber temperature was measured with a microthermocouple. The irradiated teeth were evaluated by Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM).

Results: The pulp chamber temperature rise ranged from 0.5 to 19° C depending on the location of the window and distance to pulp chamber. SEM revealed crystal fusion in both enamel and dentin at all cumulative fluences. At cumulative fluences of 40 J/cm², 200 pulses/second and higher, measurable tissue loss was observed with PLM both in dentin and enamel.

Conclusions: These results indicate there are threshold conditions above which pulsed CO₂ laser light used for soft tissue surgery may cause detrimental changes to underlying oral hard tissue and to the pulp.     

Evaluation of different therapeutic Carnoy's formulations on hard human tissues: A Raman microspectroscopy,microhardness, and scanning electron microscopy study

Purpose

To evaluate different therapeutic Carnoy's solution formulations on hard human tissues.

Effect of silver and fluoride ions on enamel demineralization: a quantitative study using micro-computed tomography

Background: This study aimed to investigate the effect of silver and fluoride ions on demineralization of enamel. Methods: The coronal parts of 40 extracted sound premolars were prepared into tooth blocks. An unvarnished occlusal surface window (OW) and a flat buccal/lingual surface window (FW) were created for each tooth by covering all other surfaces with an acid‐resistant varnish. These blocks were randomly allocated into four groups of 10 blocks each and immersed in respective solutions for 5 minutes: Group 1 – 2.36 M silver fluoride; Group 2 – 2.36 M potassium fluoride; Group 3 – 2.36 M silver nitrate; and Group 4 – deionized water. After 7‐day immersion in a buffered demineralization solution at pH 4.4, micro‐CT scans were taken. Results: Mean lesion depth in the FW area for tooth blocks in AgF, KF, AgNO₃ and control groups were 0 μm, 3.3 ± 10.3 μm, 156.3 ± 30.8 μm, and 173.6 ± 48.6 μm, respectively (p < 0.001). The difference in mean lesion depth between the AgNO₃ and control groups was not statistically significant (p > 0.05). Similar OW and FW lesions were observed in tooth blocks in the AgNO₃ and control groups. Conclusions: Topical application of a 2.36 M fluoride solution can inhibit demineralization of enamel while topical application of silver ions has little effect.