Rebecca Newhouse

High-sensitivity molecular sensing using hollow-core photonic crystal fiber and surface-enhanced Raman scattering.

A high-sensitivity molecular sensor using a hollow-core photonic crystal fiber (HCPCF) based on surface-enhanced Raman scattering (SERS) has been experimentally demonstrated and theoretically analyzed. A factor of 100 in sensitivity enhancement is shown in comparison to direct sampling under the same conditions. With a silver nanoparticle colloid as the SERS substrate and Rhodamine 6G as a test molecule, the lowest detectable concentration is 10(-10) M with a liquid-core photonic crystal fiber (LCPCF) probe, and 10(-8) M for direct sampling. The high sensitivity provided by the LCPCF SERS probe is promising for molecular detection in various sensing applications.

Highly reproducible synthesis of hollow gold nanospheres with near infrared surface plasmon absorption using PVP as stabilizing agent

An improved synthetic method has been designed and demonstrated to reproducibly generate hollow gold nanospheres (HGNs) with strong surface plasmon resonance (SPR) absorption in the near infrared (NIR). The HGNs have been synthesized via galvanic replacement of cobalt with gold while utilizing different amounts of poly(vinylpyrrolidone) (PVP) as a template stabilizing agent. Ninety percent of syntheses performed by this modified method resulted in HGNs with an SPR near 800 nm, which is highly desirable for biomedical applications such as photothermal ablation (PTA) therapy, while other polymers (PAA and PEG) did not. Based on absorption and TEM measurements, PVP stabilizes the cobalt template particles via carbonyl-induced stabilization that slows nucleation and growth of the gold shell allowing for the generation of a reproducibly thin shell, thereby inducing a significant red shift of the SPR to 800 nm. The results are significant to various potential applications of HGNs, e.g. cancer therapy and sensing.

Inner wall coated hollow core waveguide sensor based on double substrate surface enhanced Raman scattering

A hollow core waveguide with silver nanoparticles coated on the inner wall has been used for molecular detection based on surface enhanced Raman scattering (SERS). With rhodamine 6G as an analyte molecule and two types of silver nanoparticles as double SERS substrates, the inner wall coated hollow core waveguide (IWCHCW) exhibits higher sensitivity than previous sampling methods with only one substrate. The improvement of sensitivity is attributed to the additional enhancement of the electromagnetic field by double substrate “sandwich” structure. The simple architecture and high sensitivity of IWCHCW make it promising for molecular detection in various analytical and sensing applications.

Hollow-Core Photonic Crystal Fibers for Surface-Enhanced Raman Scattering Probes

Photonic crystal fiber (PCF) sensors based on surface-enhanced Raman scattering (SERS) have become increasingly attractive in chemical and biological detections due to the molecular specificity, high sensitivity, and flexibility. In this paper, we review the development of PCF SERS sensors with emphasis on our recent work on SERS sensors utilizing hollow-core photonic crystal fibers (HCPCFs). Specifically, we discuss and compare various HCPCF SERS sensors, including the liquid-filled HCPCF and liquid-core photonic crystal fibers (LCPCFs). We experimentally demonstrate and theoretically analyze the high sensitivity of the HCPCF SERS sensors. Various molecules including Rhodamine B, Rhodamine 6G, human insulin, and tryptophan have been tested to show the excellent performance of these fiber sensors.

Portable fiber sensors based on surface-enhanced Raman scattering

Two portable molecular sensing systems based on surface-enhanced Raman scattering (SERS) have been experimentally demonstrated using either a tip-coated multimode fiber (TCMMF) or a liquid core photonic crystal fiber (LCPCF) as the SERS probe. With Rhodamine 6G as a test molecule, the TCMMF-portable SERS system achieved 2-3 times better sensitivity than direct sampling (focusing the laser light directly into the sample without the fiber probe), and a highly sensitive LCPCF-portable SERS system reached a sensitivity up to 59 times that of direct sampling, comparable to the sensitivity enhancement achieved using fiber probes in the bulky Renishaw system. These fiber SERS probes integrated with a portable Raman spectrometer provide a promising scheme for a compact and flexible molecular sensing system with high sensitivity and portability.

Ultrafast Exciton Dynamics in Silicon Nanowires.

Ultrafast exciton dynamics in one-dimensional (1D) silicon nanowires (SiNWs) have been investigated using femtosecond transient absorption techniques. A strong transient bleach feature was observed from 500 to 770 nm following excitation at 470 nm. The bleach recovery was dominated by an extremely fast feature that can be fit to a triple exponential with time constants of 0.3, 5.4, and ∼75 ps, which are independent of probe wavelength. The amplitude and lifetime of the fast component were excitation intensity-dependent, with the amplitude increasing more than linearly and the lifetime decreasing with increasing excitation intensity. The fast decay is attributed to exciton-exciton annihilation upon trap state saturation. The threshold for observing this nonlinear process is sensitive to the porosity and surface properties of the sample. To help gain insight into the relaxation pathways, a four-state kinetic model was developed to explain the main features of the experimental dynamics data. The model suggests that after initial photoexcitation, conduction band (CB) electrons become trapped in the shallow trap (ST) states within 0.5 ps and are further trapped into deep trap (DT) states within 4 ps. The DT electrons finally recombine with the hole with a time constant of ∼500 ps, confirming the photophysical processes to which we assigned the decays.

Fiber sensors for molecular detection

The demand on sensors for detecting chemical and biological agents is greater than ever before, including medical, environmental, food safety, military, and security applications. At present, most detection or sensing techniques tend to be either non-molecular specific, bulky, expensive, relatively inaccurate, or unable to provide real time data. Clearly, alternative sensing technologies are urgently needed. Recently, we have been working to develop a compact fiber optic surface enhanced Raman scattering (SERS) sensor system that integrates various novel ideas to achieve compactness, high sensitivity and consistency, molecular specificity, and automatic preliminary identification capabilities. The unique sensor architecture is expected to bring SERS sensors to practical applications due to a combination of 1) novel SERS substrates that provide the high sensitivity and consistency, molecular specificity, and applicability to a wide range of compounds; 2) a unique hollow core optical fiber probe with double SERS substrate structure that provides the compactness, reliability, low cost, and ease of sampling; and 3) an innovative matched spectral filter set that provides automatic preliminary molecule identification. In this paper, we will review the principle of operation and some of the important milestones of fiber SERS sensor development with emphasis on our recent work to integrate photonic crystal fiber SERS probes with a portable Raman spectrometer and to demonstrate a matched spectral filter for molecule identification.

Optical Properties and Applications of Shape-Controlled Metal Nanostructures

Noble metallic materials in the nanoscale size regime display unique optical and electronic properties. In this chapter, we review current synthetic methods for making gold and silver nanostructures of various shapes and sizes and discuss select techniques that have become indispensible for structural characterization of nanomaterials. A fundamental characteristic of metal nanostructures is their interaction with light through surface plasmon resonance (SPR), which can be modulated by shape, surrounding environment, and size. We first address the effect of shape on the SPR and then discuss changes in the extinction spectrum that result from plasmonic coupling between metal nanostructures in close proximity. Changes in the refractive index of media surrounding the nanoparticles and the corresponding effect on the SPR are briefly addressed. We then highlight several applications of metal nanostructures that utilize their unique optical properties, including SPR as a technique for detection, surface-enhanced Raman scattering (SERS), photothermal ablation therapy (PTA), drug delivery, and solar energy conversion. We conclude with a discussion of current challenges and promising research directions in the study and use of metal nanostructures.

Temperature-dependent Raman scattering study of LiAlH4 and Li3AlH6

A temperature dependent Raman scattering study was conducted for the first time on LiAlH4 and Li3AlH6 in an effort to monitor the reaction kinetics of these hydrides toward its application as a hydrogen storage/delivery system for fuel cell and other applications. LiAlH4 demonstrated a gradual softening of its Raman modes as the temperature was increased from 25 to 145 °C while Li3AlH6 demonstrated a loss in its Raman signal as the temperature increased from 30 to 90 °C. The more sensitive Raman response of Li3AlH6 is believed to be the result of the weaker attraction between the octahedral AlH63- units in the anionic network of the Li3AlH6 compared to the tetrahedral AlH4- units of LiAlH4.

Mix and match: enhanced Raman spectroscopy instrumentation in field applications

We present Raman spectroscopy analysis on laboratory and field sample analysis on several expeditions. Our measurements in mineral and organic composition have demonstrated that both mineral and organic species in low concentrations can be identified with Raman spectroscopy with no sample preparations and without instrument probe contact to the samples. Our laboratory studies on cyanobacterial biomat, and Mojave Desert rocks have demonstrated the promising potential for Raman spectroscopy as a nondestructive, in situ, high throughput detection technique, as well as a desirable active remote sensing tool for future planetary and space missions.