Konrad Maier

Detection of Low-Concentration Contaminants in Solution by Exploiting Chemical Derivatization in Surface-Enhanced Raman Spectroscopy

A simple derivatization methodology is shown to extend the application of surface-enhanced Raman spectroscopy (SERS) to the detection of trace concentration of contaminants in liquid form. Normally in SERS the target analyte species is already present in the molecular form in which it is to be detected and is extracted from solution to occupy sites of enhanced electromagnetic field on the substrate by means of chemisorption or drop-casting and subsequent evaporation of the solvent. However, these methods are very ineffective for the detection of low concentrations of contaminant in liquid form because the target (ionic) species (a) exhibits extremely low occupancy of enhancing surface sites in the bulk liquid environment and (b) coevaporates with the solvent. In this study, the target analyte species (acid) is detected via its solid derivative (salt) offering very significant enhancement of the SERS signal because of preferential deposition of the salt at the enhancing surface but without loss of chemical discrimination. The detection of nitric acid and sulfuric acid is demonstrated down to 100 ppb via reaction with ammonium hydroxide to produce the corresponding ammonium salt. This yields an improvement of ~4 orders of magnitude in the low-concentration detection limit compared with liquid phase detection.

Photoluminescence Probing of Complex H2O Adsorption on InGaN/GaN Nanowires

We demonstrate that the complex adsorption behavior of H2O on InGaN/GaN nanowire arrays is directly revealed by their ambient-dependent photoluminescence properties. Under low-humidity, ambient-temperature, and low-excitation-light conditions, H2O adsorbates cause a quenching of the photoluminescence. In contrast, for high humidity levels, elevated temperature, and high excitation intensity, H2O adsorbates act as efficient photoluminescence enhancers. We show that this behavior, which can only be detected due to the low operation temperature of the InGaN/GaN nanowires, can be explained on the basis of single H2O adsorbates forming surface recombination centers and multiple H2O adsorbates forming surface passivation layers. Reversible creation of such passivation layers is induced by the photoelectrochemical splitting of adsorbed water molecules and by the interaction of reactive H3O+ and OH- ions with photoactivated InGaN surfaces. Due to electronic coupling of adsorbing molecules with photoactivated surfaces, InGaN/GaN nanowires act as sensitive nanooptical probes for the analysis of photoelectrochemical surface processes.

III-nitride nanostructures for optical gas detection and pH sensing

The paper presents a novel concept for the realization of optochemical sensor systems which are capable of operating in harsh environments. Key components in such sensors are nanostructures formed from gallium nitride (GaN) and its alloys with aluminum (Al) and indium (In). Nanostructures of this kind emit an efficient, visible-light photoluminescence (PL) which can be excited with low-cost ultraviolet light sources and which extends up to temperatures in the order of 200°C. When exposed to various chemical environments, changes in the PL intensity occur which constitute valuable sensor signals. Due to the all-optical approach, the PL can be excited and its chemically induced changes be read out without requiring electrical wiring at the point of measurement. The present paper presents this innovative sensor concept, the nanostructures and optochemical transducer structures that form its material base, as well as several applications of such transducers in the fields of gas and fluid sensing. The applications addressed here range from the sensing of ppb concentrations of H2, NO2 and O3 in gaseous environments to the pH monitoring in aqueous solutions.