Ulrich Nickel

REPRODUCIBLE PREPARATION OF SILVER SOLS WITH UNIFORM PARTICLE SIZE FOR APPLICATION IN SURFACE-ENHANCED RAMAN SPECTROSCOPY

Abstract This paper describes a new method for the preparation of silver colloids with a narrow range of particle size to be used in surface‐enhanced Raman spectroscopy. Using malachite green as a strongly adsorbing dye, it can be shown that colloids from different preparation batches exhibit the same enhancement factor within an error margin of about 15%. By varying the number of nucleation centers, the particle size can be determined at will. An increase in particle diameter from about 38 to about 76 nm leads to an estimated five‐fold increase in surface enhancement.

Stabilization of Silver Colloids by Various Types of Anions and Their Effect on the Surface‐Enhanced Raman Spectra of Organic Dyes

Using a recently published procedure, silver colloids with uniform particle size were prepared and stabilized by nine different anions. Although the UV–visible extinction spectra of the colloids showed only small variations with the nature of the stabilizing ions, dramatic differences were observed in the SERS spectra of seven dyes studied. It was demonstrated that each family of dyes experiences a large enhancement factor only with certain stabilizing ions. The adsorption geometry of oxazine dyes changes with the nature of the stabilizing anion and with the concentration of the dye. By comparison of observed and calculated normal modes, conclusions about the adsorption geometry are drawn. The results are discussed in the framework of the ‘adatom model’.

Preparation of SERS-active silver film electrodes via electrocrystallization of silver

The electrodeposition of a sufficiently large amount of silver on silver films at high over potentials leads to excellent substrates for surface-enhanced Raman scattering (SERS) spectroscopy. The surfaces roughened in this manner show great uniformity across the active area and exhibit SERS enhancement factors which are superior to those of silver surfaces roughened by conventional oxidation–reduction cycles. Since they are excellently reproduced from one sample to another, the prospects for application of this substrate for analytical purposes are very promising. Copyright

Kinetic effects in surface-enhanced Raman spectroscopy: does it have potential as an analytical tool?

The intensity of surface-enhanced Raman spectra, in general, and the relative intensities of different bands, in particular, depend on many parameters and are usually subject to variation with respect to standing time. Previously, several attempts have been made to explain the physical background to some of these variations, but so far no generally valid explanation has been proposed. As a consequence, surface-enhanced Raman spectroscopy, despite its extremely high sensitivity, has not yet been accepted as an analytical tool. The work described in this paper demonstrates that an analytical application is possible under certain conditions.

Enhancement of an insufficient dye-formation in the ninhydrin reaction by a suitable post treatment process.

A study of the reaction between ninhydrin and alanine has been carried out taking into account that, adhered on the surface of a dry porous medium such as paper, a quasi-heterogeneous reaction has to take place. Instead of amino acids released from human sweat glands, aqueous solutions of alanine were taken to deliver a given amount of amino acid to the sample. The dye density, obtained after using a standard development process, could noticeably be increased by setting a drop of water on the dye dot, thus indicating that not all the alanine had been used for dye formation during the usually applied process. The incomplete reaction can be explained by the problem of bringing the reactants into contact with each other when both are in the solid phase in the porous surrounding. The temporary presence of water allows a reorientation of the insoluble reactants. With fingerprints an increase in both the rate of development and the final dye density could be obtained when the sample was post treated after the developing process with the blank solvent, thus also the background coloration could be decreased. The ideas presented in this paper may form the basis for a modification of developing processes with ninhydrin in order to increase the proportion of amino acids present (in the sample) used in dye formation without data loss.

Transdermal Delivery Using a Novel Electrochemical Device, Part 1: Device Design and In Vitro Release/Permeation of Fentanyl

A new type of electrochemical transdermal patch has been investigated using release/permeation experiments with excised nude mouse skin. The patch comprises a drug-containing hydrogel sandwiched between two electrodes that are arranged parallel to the skin surface. The objective was to determine the mechanism of working of enhanced flux for the drug fentanyl when low voltages are applied. The results indicate that a voltage-induced hydrolysis of the water present in the patchs hydrogel occurs. This causes a pH shift that results in deprotonization of the fentanyl and hence enhanced release/permeation. The enhanced flux is up to approximately 30 μg/(cm(2) h) over 20 h and requires only a low-voltage application over a duration of just 60 s. Because the enhancement mechanism occurs in the patch and not in the skin, the potential for substantially reduced skin irritation compared with iontophoresis is given.

Transdermal Delivery Using a Novel Electrochemical Device, Part 2: In Vivo Study in Humans

A single-center, open-label, and dose-escalation clinical trial has been performed using a novel electrochemical transdermal patch comprising a drug-loaded hydrogel sandwiched between two silver electrodes. The lower electrode is attached flatly to skin via an adhesive layer. This noniontophoretic device produces flux enhancement via voltage-induced electrolysis of the water in the hydrogel. A voltage application of 2 V over 60 s produced release of approximately 315 to 340 μg of fentanyl from a patch. A single-voltage application at t = 16 h produced a maximum plasma level of approximately 200 pg/mL before patch removal at t = 41 h. Consecutive voltage applications at t = 16 h and t = 40 h produced a maximum plasma level of approximately 730 pg/mL. The lag time between voltage application and achievement of a plasma profile plateau is longer than that reported for an iontophoretic patch. The patchs advantage is production of an on-demand plasma profile without the use of potentially irritating iontophoresis. In contrast to a passive transdermal system, it is possible to adjust the plasma profile by using voltage application. It is not necessary to apply a continual current, in contrast to iontophoretic systems.

The Influence of Halide on the Silver-Catalysed Oxidation of N,N-Dimethyl-p-phenylenediamine by [CoIII(NH3)5Cl]2+

AbstractThe mechanism of the silver colloid catalysed oxidation of N,N-dimethyl-p-phenylenediamine by CoIII(NH3)5Cl2+ was studied in the presence of halides in aqueous solution at a pH of about 5. The silver colloids were prepared by reduction of silver nitrate with ascorbic acid or with sodium boranate in the absence of a special stabilizing agent. The course of the reaction could easily be followed in a double wavelength photometer because of the strong absorbance of p- semiquinonediimine, the first oxidation product of p-phenylenediamine. Both the reaction rate and the irreversible course of the reaction depend on the kind and the concentration of halide present in the solution. Apart from the electrocatalytic redox reaction, the oxidative formation of silver halide by cobalt (III) and the reduction of this silver halide by p-phenylenediamine occur simultaneously. Therefore, the mechanism of at least three important photographic processes can be studied at the same time in this redox system.

Kinetics and mechanism of the acid deamination of N-substituted quinone di-imines measured with a multi-mixing, stopped-flow technique

A stopped flow apparatus with six syringes and five mixing chambers is described which allows several stoppages at given intervals. Seven partially ring methylated, N-substituted p-phenylenediamines (R) were oxidised by iodine or permanganate. The deamination of the quinone di-imines (T+) formed was studied in the pH range 1–10 by means of a colour coupling process after different intervals or from the extinction of the radical intermediate as a function of time. T+ is in a fast equilibrium with its protonated form TH2+. Deamination is initiated by nucleophilic attack on the most positively charged carbon atom of the ring. Therefore, in acid solution TH2+ is deaminated by H2O giving the substituted quinone monoimine and in alkaline solution T+ is deaminated by OH–, giving the respective unsubstituted quinone monoimine. The equilibria and the rate constants were measured at 25 °C. The stability maximum of T+ is at pH 6 whereas the N-substituted quinone monoimines are stable at pH <2. The further deamination of quinone monodimethylimine was studied between pH 1 and 10. Parallel reactions with OH– and with H2O were found, giving in both cases p-benzoquinone

Cyclic voltammetry of N,N-dimethyl-p-phenylenediamine and its HCl-salt in acetonitrile with pyridine as proton acceptor

Summary The redox behaviour of N,N-dimethyl-p-phenylenediamine (PPD) and N,N-dimethyl-p-phenylenediamine · 2HCl (PPD · 2HCl) in acetonitrile was studied by means of cyclic voltammetry – both in the absence and in the presence of proton accepting pyridine. The oxidation occurs in two separate steps yielding first N,N-dimethyl-p-semiquinonediimine (SQDI+) and subsequently N,N-dimethyl-p-quinonediimine (QDI2+ or QDI+(H+)). In case of N,N-dimethyl-p-phenylenediamine the 1st oxidation step is fully reversible, while the 2nd oxidation step is slightly quasi-reversible. At 10°C the formal redox potentials of the PPD/SQDI+ and SQDI+/QDI2+ redox couples are −0.200 ± 0.005 V and 0.380 ± 0.010 V (versus Fc/Fc+), respectively. The synproportionation constant Ksyn = [SQDI+]2/[PPD][QDI2+] is 2.1 × 1010. Adding pyridine to the reaction mixture leads to a shift of the 2nd anodic wave to less positive potentials. This shift is primarily due to proton transfer from QDI2+ to pyridine to form QDI+, which overall renders the oxidation easier. The corresponding cathodic peak, on the other hand, disappears as the reduction of QDI+ necessitates a proton. The formal oxidation potential of the redox couple SQDI+/QDI+(H+) is about 0.190 V and, therefore, the synproportionation constant Ksyn = 1.7 × 106. N,N-dimethyl-p-phenylenediamine · 2HCl lacks completely the 1st anodic peak. The position of the anodic peak is given by the direct oxidation of PPDH+ to QDI2+(H+) and is similar to that of the SQDI+/QDI2+ redox couple. Upon adding pyridine the 2nd anodic peak shifts notably to less positive potentials and the redox process attributable to the PPD/SQDI+ redox couple is seen. As pyridine influences the potential of the Ag pseudo-reference electrode a reference electrode based on the ferrocene/ferrocenium redox couple was used.