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43879-G10
Single Molecule Raman Detection with a Composite Microresonator and Metal Nanocavity System
Surface enhanced Raman spectroscopy (SERS) is a widely-studied technique capable of adding single-molecule detection capability to the rich information provided by Raman spectroscopy. In the proposal, we proposed to combine the whispering gallery modes (WGMs) of ring resonator with nanocavities formed by metallic nanoparticle cluster to achieve multiplicative enhancement in Raman signal that enables single molecule Raman detection.
We have so far performed experiments on two systems, microsphere based ring resonators, which is originally proposed in the proposal, and liquid core optical ring resonator (LCORR), which is a newly developed ring resonator invented in our lab. In a microsphere system, silver clusters consisting of silver nanoparticles are coated on the sphere exterior surface and the resonant light is coupled into the sphere and excites the SERS. With this system, a factor of 300 enhancement has been achieved, in addition to the conventional SERS, which enhances Raman signal one to one hundred million times on average.
The LCORR utilizes a small glass capillary as a sample delivery mechanism and as an optical ring resonator. The hollow core of the capillary carries the sample while the circular cross-section acts as the ring resonator. In contrast to the microsphere case where the WGM utilizes its evanescent field on the sphere exterior surface, the LCORR employs the evanescent field at its interior surface for SERS excitation. Therefore, the silver nanoclusters are coated on the LCORR interior surface. The LCORR is very similar to the microsphere in terms of optical performance and SERS enhancement. However, the LCORR is much advantageous in terms fluidics, as it naturally integrates the microfluidics with ring resonator. Unlike the microsphere, no additionally fluidics is needed. Our report will then focus on the LCORR based SERS enhancement.
The LCORR relies on a thin-walled capillary with an outer diameter of approximately 100 micrometers and wall thickness of a few micrometers. Since commercialized thin-walled capillary is not available, we have assembled a computer-controlled pulling station to fabricate the LCORR up to 50 cm long from a preform. The silver colloids are produced with the reduction method introduced by Lee and Meisel. To promote clustering, NaCl is added to the colloid. Rhodamine 6G (R6G) is used as the target analyte because it has a well-characterized Raman signal and because it is commonly used in many previously published works on SERS. R6G dissolved in water is added to each colloid aliquot, which is subsequently pumped through the LCORR using a syringe pump. The SERS enhancement of the colloid at 785 nm is tested by measuring the Raman signal of various concentrations of R6G in silver colloid aliquots and comparing the results with the Raman signal measured from R6G solutions in water. The average enhancement is found to be nearly one million, which is excellent at 785 nm illumination. In reality, the enhancement for individual R6G molecules adsorbed onto silver clusters is likely to be at least one order of magnitude higher, as most of the R6G molecules for the given concentration are not adsorbed onto silver clusters and thus do not contribute to the measured Raman signal. Besides one million times SERS enhancement, the LCORR provides additional one thousand times enhancement due to the cavity effect. So far, we have achieved a measured detection limit of 400 pM in R6G, which is significantly lower than other reported detection limits (10 nM - 1 uM) for microfluidic-based SERS designs. The measured Raman signal in our experiment is likely generated by only a few hundred R6G molecules, which foreshadows the development of a SERS-based lab-on-a-chip bio/chemical sensor capable of detecting a low number of target analyte molecules.
Our immediate next step is to improve the Q-factor of the LCORR and light collection efficiency of our setup in order to detect an even lower the Raman signal. Furthermore, we plan to initiate correlation spectroscopy measurement that can tell whether or not the detected Raman signal is generated by a single molecule.
