Martin J. T. Milton

New limits in ultrasensitive trace detection by surface enhanced Raman scattering (SERS)

We show the possibility of chemical tracing in solution down to a concentration of a fraction of an atto mol (10 � 18 ) by means of non-resonant surface enhanced Raman scattering (SERS). The experiments are demonstrated with rhodamine 6G dissolved in water. Based on our results here, we propose a technique for counting a small number of SERS active molecules in micro-droplets of liquid. It is argued that, by choosing the appropriate SERS enhancement condition, the technique could be used and implemented as a routine analytical method with unprecedented detection capabilities for the monitoring and tracing of important families of biomolecules. 2003 Elsevier Science B.V. All rights reserved.

Electromagnetic contribution to surface enhanced Raman scattering revisited

Several aspects of the electromagnetic contribution to surface enhanced Raman scattering (SERS) are revisited and analyzed in terms of known concepts and ideas in the field of photonic crystals. A qualitative understanding of some reported SERS phenomena can be achieved by this analysis of the complex energy distribution and spatial localization of surface plasmon resonances in SERS active media. A few of the outstanding issues in the electromagnetic contribution to SERS (like the overwhelming success of nonresonant excitation) can be explained within this framework and, in addition, new physical phenomena utilizing the stimulation or inhibition of the Stokes/anti-Stokes fields are proposed.

The physical and chemical properties of electroless nickel–phosphorus alloys and low reflectance nickel–phosphorus black surfaces

Novel insights into the manufacture of nickel–phosphorus black surfaces by chemical etching of electroless-deposited Ni–P alloy has been achieved by examining the influence of pre-etch phosphorus composition and etching method on the resulting morphology, composition and reflectance of the black surface produced. An optimum phosphorus composition and etching regime to produce low reflectance blacks of 0.4% or lower in the visible region is proposed. Cross-sectional analysis of the etched surface has allowed, for the first time, an accurate determination of the scale of the enhanced morphologies produced and the thickness of the oxidised black layer itself. AFM studies have also provided information on the phase structure of the as-deposited Ni–P alloy.

Microfabricated planar glass gas chromatography with photoionization detection

We report the development of a microfabricated gas chromatography system suitable for the separation of volatile organic compounds (VOCs) and compatible with use as a portable measurement device. Hydrofluoric acid etching of 95x95mm Schott B270 wafers has been used to give symmetrical hemi-spherical channels within a glass substrate. Two matching glass plates were subsequently cold bonded with the channels aligned; the flatness of the glass surfaces resulted in strong bonding through van der Waals forces. The device comprised gas fluidic interconnections, injection zone and 7.5 and 1.4m long, 320microm internal diameter capillaries. Optical microscopy confirmed the capillaries to have fully circular channel profiles. Direct column heating and cooling could be achieved using a combination of resistive heaters and Peltier devices. The low thermal conductivity of glass allowed for multiple uniform temperature zones to be achieved within a single glass chip. Temperature control over the range 10-200 degrees C was achieved with peak power demand of approximately 25W. The 7.5m capillary column was static coated with a 2microm film of non-polar dimethylpolysiloxane stationary phase. A standard FID and a modified lightweight 100mW photoionization detector (PID) were coupled to the column and performance tested with gas mixtures of monoaromatic and monoterpene species at the parts per million concentration level. The low power GC-PID device showed good performance for a small set of VOCs and sub ng detection sensitivity to monoaromatics.

Observation of dynamic oxygen release in hemoglobin using surface enhanced Raman scattering

Photoinduced oxygen dynamics in hemoglobin is monitored by surface enhanced resonant Raman spectroscopy (SERRS) using silver colloids. The spectra are characterized by massive transient amplifications which we assign to a charge-transfer mechanism during the opening of the heme-pockets in the release of oxygen. This transient lifts the selection rule specificity of normal Raman active modes and allows for the full density of states of the hemes to be observed. The amplification of specific forbidden modes is controlled by the orientation alignment of individual molecules in proximity with the colloid surface. We propose that the technique can be used to monitor oxygen release in single proteins.

Gravimetric methods for the preparation of standard gas mixtures

The most widely used method for the preparation of primary standard gas mixtures involves weighing the individual components into a cylinder. We present a new mathematical description of the method and its uncertainties. We use this to demonstrate how strategies for serial dilution can be identified that minimize the uncertainty in the final mixture and show how they can be implemented practically. We review published reports of high accuracy gravimetry and give examples of relative uncertainties in the composition of standards approaching 1 part-per-million in the best cases and in the range of 100 to 1000 parts-per-million more typically.

Electromagnetic modelling of Raman enhancement from nanoscale substrates: a route to estimation of the magnitude of the chemical enhancement mechanism in SERS

Despite widespread use for more than two decades, the SERS phenomenon has defied accurate physical and chemical explanation. The relative contributions from electronic and chemical mechanisms are difficult to quantify and are often not reproduced under nominally similar experimental conditions. This work has used electromagnetic modelling to predict the Raman enhancement expected from three configurations: metal nanoparticles, structured metal surfaces, and sharp metal tips interacting with metal surfaces. In each case, parameters such as artefact size, artefact separation and incident radiation wavelength have been varied and the resulting electromagnetic field modelled. This has yielded an electromagnetic description of these configurations with predictions of the maximum expected Raman enhancement, and hence a prediction of the optimum substrate configuration for the SERS process. When combined with experimental observations of the dependence of Raman enhancement with changing ionic strength, the modelling results have allowed a novel estimate of the size of the chemical enhancement mechanism to be produced.

A novel amplification mechanism for surface enhanced Raman scattering

Abstract Macromolecules in contact with both metallic colloids and oxygen may display unusually large dynamic hot-spots for surface enhanced Raman spectroscopy (SERS). The effect has been shown to exist in systems as different as proteins (hemoglobin) and carbon chain segments, but we also show here its existence in SERS of single-stranded DNA. The physicochemical origin of this effect is inquired into, and a specific model based on resonant charge–transfer interactions (mediated by oxygen) between the surface plasmons and the molecules is proposed as a microscopic origin. It is argued that, by a proper choice of lasers, the effect could be used for biomolecules to detect traces down to a single-molecule level.

Pulse averaging methods for a laser remote monitoring system using atmospheric backscatter

Three methods for averaging differential absorption lidar (DIAL) measurements are discussed. They are compared using experimental data acquired with a dual-laser direct-detection DIAL system operating in the near UV using atmospheric backscatter. The data set was acquired with the two lasers tuned to the same wavelength to eliminate any spectral variations and fluctuations of differential absorbers from the measurement. The results are compared by evaluating both the standard deviations and the means of the integrated columns. The results suggest that shot noise on the backscattered signals dominates the measurement statistics and considerably reduces the necessity for short interpulse delay times.

Amount of substance and the proposed redefinition of the mole

There has been considerable discussion about the merits of redefining four of the base units of the SI, including the mole. In this paper, the options for implementing a new definition for the mole based on a fixed value for the Avogadro constant are discussed. They are placed in the context of the macroscopic nature of the quantity amount of substance and the opportunity to introduce a system for molar and atomic masses with unchanged values and consistent relative uncertainties.