Design of a simple cryogenic system for ultraviolet-visible absorption spectroscopy with a back-reflectance fiber optic probe.

We report a convenient and inexpensive technique for the rapid acquisition of absorption spectra from small samples at cryogenic temperatures using a home built cryostat with novel collection optics. A cylindrical copper block was constructed with a coaxial bore to hold a 4.00 mm diameter electron paramagnetic resonance (EPR) tube and mounted on a copper feed in thermal contact with liquid nitrogen. A 6.35 mm diameter hole was bored into the side of the cylinder so a fiber optic cable bundle could be positioned orthogonally to the EPR tube. The light passing through the sample is reflected off of the opposing surfaces of the EPR tube and surrounding copper, back through the sample. The emergent light is then collected using the fiber optic bundle and analyzed using a dispersive spectrometer. Absorption spectra for KMnO4 were measured between 400 and 700 nm. Absorption intensity at 506, 525, 545, and 567 nm was found to be proportional to concentration, displaying Beer's law-like behavior. The EPR tube had an internal diameter of 3.2 mm; the double pass of the probe beam through the sample affords a central path length of about 6.4 mm. Comparing these measurements with those recorded on a conventional tabletop spectrometer using a cuvette with a 10.00 mm path length, we consistently found a ratio between intensities of 0.58 rather than the anticipated 0.64. These 6% smaller values we attribute to the curvature of the EPR tube and transmission/reflection losses. This system is particularly well-suited to studying the kinetics and dynamics of chemical reactions at cryogenic temperatures. The rapid response (100 ms) and multiplex advantage provided the opportunity of recording simultaneous time courses at several wavelengths following initiation of a chemical reaction with a pulsed laser source.

In vivo glucose measurement by surface-enhanced Raman spectroscopy.

This paper presents the first in vivo application of surface-enhanced Raman scattering (SERS). SERS was used to obtain quantitative in vivo glucose measurements from an animal model. Silver film over nanosphere surfaces were functionalized with a two-component self-assembled monolayer, and subcutaneously implanted in a Sprague-Dawley rat such that the glucose concentration of the interstitial fluid could be measured by spectroscopically addressing the sensor through an optical window. The sensor had relatively low error (RMSEC = 7.46 mg/dL (0.41 mM) and RMSEP = 53.42 mg/dL (2.97 mM).

Surface enhanced Raman spectroscopy: new materials, concepts, characterization tools, and applications.

Surface-enhanced Raman spectroscopy (SERS) is currently experiencing a renaissance in its development driven by the remarkable discovery of single molecule SERS (SMSERS) and the explosion of interest in nanophotonics and plasmonics. Because excitation of the localized surface plasmon resonance (LSPR) of a nanostructured surface or nanoparticle lies at the heart of SERS, it is important to control all of the factors influencing the LSPR in order to maximize signal strength and ensure reproducibility. These factors include material, size, shape, interparticle spacing, and dielectric environment. All of these factors must be carefully controlled to ensure that the incident laser light maximally excites the LSPR in a reproducible manner. This article describes the use of nanosphere lithography for the fabrication of highly reproducible and robust SERS substrates for both fundamental studies and applications. Atomic layer deposition (ALD) is introduced as a novel fabrication method for dielectric spacers to study the SERS distance dependence and control the nanoscale dielectric environment. Wavelength scanned SER excitation spectroscopy (WS SERES) measurements show that enhancement factors approximately 10(8) are obtainable from NSL-fabricated surfaces and provide new insight into the electromagneticfield enhancement mechanism. Tip-enhanced Raman spectroscopy (TERS) is an extremely promising new development to improve the generality and information content of SERS. A 2D correlation analysis is applied to SMSERS data. Finally, the first in vivo SERS glucose sensing study is presented.

Surface-enhanced Raman spectroscopy of half-mustard agent.

The detection and identification of chemical warfare agents is an important analytical goal. Herein, it is demonstrated that 2-chloroethyl ethyl sulfide (half-mustard, CEES) can be successfully analysed using surface-enhanced Raman spectroscopy (SERS). A critical component in this detection system is the fabrication of a robust, yet highly enhancing, sensor surface. Recent advances in substrate fabrication and in the fundamental understanding of the SERS phenomenon enable the development of improved substrates for practical SERS applications.

Glucose sensing using near-infrared surface-enhanced Raman spectroscopy: gold surfaces, 10-day stability, and improved accuracy.

This research presents the achievement of significant milestones toward the development of a minimally invasive, continuously monitoring, glucose-sensing platform based on the optical quantitation of glucose in interstitial fluid. We expand our initial successes in the measurement of glucose by surface-enhanced Raman scattering (SERS), demonstrating substantial improvements not only in the quality and optical properties of the substrate system itself but also in the robustness of the measurement methodology and the amenability of the technique to compact, diode laser-based instrumentation. Herein, we compare the long-term stability of gold to silver film over nanosphere (AuFON, AgFON) substrates functionalized with a partitioning self-assembled monolayer (SAM) using both electrochemical and SERS measurements. AuFONs were found to be stable for a period of at least 11 days. The switch to AuFONs not only provides a more stable surface for SAM formation but also yields better chemometric results, with improved calibration and validation over a range of 0.5-44 mM (10-800 mg/dL). Measured values for glucose concentrations in phosphate-buffered saline (pH approximately 7.4) based on 160 independent SERS measurements on AuFONs have a root-mean-square error of prediction of 2.7 mM (49.5 mg/dL), with 91% of the values falling within an extended A-B range on an expanded Clarke error grid. Furthermore, AuFONs exhibit surface plasmon resonances at longer wavelengths than similar AgFONs, which make them more efficient for SERS at near-infrared wavelengths, enabling the use of low-power diode lasers in future devices.

Towards advanced chemical and biological nanosensors-An overview.

This paper reviews recent developments in the design and application of two types of optical nanosensor, those based on: (1) localized surface plasmon resonance (LSPR) spectroscopy and (2) surface-enhanced Raman scattering (SERS). The performance of these sensors is discussed in the context of biological and chemical sensing. The first section addresses the LSPR sensors. Arrays of nanotriangles were evaluated and characterized using realistic protein/ligand interactions. Isolated, single nanoparticles were used for chemosensing and performed comparably to the nanoparticle array sensors. In particular, we highlight the effect of nanoparticle morphology on sensing response. The second section details the use of SERS sensors using metal film over nanosphere (MFON) surfaces. The high SERS enhancements and long-term stability of MFONs were exploited in order to develop SERS-based sensors for two important target molecules: a Bacillus anthracis biomarker and glucose in a serum protein mixture.

Using solution-phase nanoparticles, surface-confined nanoparticle arrays and single nanoparticles as biological sensing platforms.

The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe three sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of gold and silver nanoparticles. Specifically, the color associated with solution-phase nanoparticles, surface-confined nanoparticle arrays, and single nanoparticles will be shown to be tunable and useful as platforms for biological sensing.