Solutions of Rhodamine 6G (R6G) in silver colloid are used as model samples in the experiments presented here. R6G is selected as the target analyte because of its well-characterized Raman spectrum and its common use in other SERS investigations. The silver colloid is prepared using the reduction method originally introduced by Lee and Meisel [18
]. To promote clustering, the silver colloid is mixed with a NaCl stock solution in water such that the silver colloid is diluted by a factor of three and the NaCl concentration is approximately 18 mM. Clustering of the silver nanoparticles is induced because individual silver nanoparticles are typically resonant for wavelengths between 400 and 500 nm, while nanoclusters have an extended resonance into the near infrared, which is used in the experiments here. Furthermore, clustering is known to induce high-enhancement “hot spots,” regions of extremely high SERS enhancement due to the nanoparticle geometry [5
]. After adding NaCl, the aliquot is shaken for 30–60 minutes, and then R6G is added to produce a concentration of 333 nM. This solution is shaken for 30–60 minutes, and then dilutions are prepared. Samples are produced shortly before the experiments are conducted.
A scanning electron microscope (SEM) image of a particularly large silver cluster is shown in . The figure shows a relatively uniform nanoparticle size of approximately 50 nm in diameter, and it shows that a high degree of clustering can occur in our samples. It is estimated that the silver nanoparticle concentration in colloid is around 2 nM, and thus is sub-nM in the NaCl activated aliquot with R6G added. Therefore, the R6G concentration is over 100 times that of the silver nanoparticles. The ratio of R6G molecules to silver clusters is even higher, and thus it is expected that many silver clusters will have several target analyte molecules. Conversely, it is expected that many target analyte molecules are not adsorbed at hot spots of the silver clusters, making the effective analyte concentration smaller than the values given here. Nonetheless, working with a very low concentration (pM or lower) of silver nanoclusters is important because it is observed that at higher concentrations, the Q-factor of the LCORR is degraded, which compromises the resulting SERS signal.
SEM of a large silver cluster within a typically prepared silver colloid solution. The solution is dried onto a TEM grid in preparation for imaging.
The samples are pumped into the LCORR using a peristaltic pump and attached tubing. The LCORR capillaries are produced from silica glass tubes as described in [19
]. The diameter of the capillaries used is approximately 125 μm. The wall thickness of the capillaries is reduced to the desired values by passing a solution of hydrofluoric acid (HF) in water through the capillary, which slowly etches the capillary wall. The thickness of the wall and the penetration of the resonant WGM evanescent field beyond the inner surface are evaluated by characterizing the refractive index (RI) sensitivity of the LCORR, as described in [11
To perform the SERS measurement, a tapered fiber optic cable with a diameter of approximately 1–2 μm is brought into contact with the LCORR, as shown in . A 785 nm DFB laser (Toptica) is coupled into the fiber taper and excites resonant modes in the LCORR. As shown in , the modes can be observed at the taper output as intensity dips when the excitation laser is repeatedly scanned in frequency. In these experiments, the Q-factor is generally above 106
, which enables many hundreds of revolutions for the average photon lifetime, as shown by the equation: N
), where N
is the average number of effective photon revolutions, R
is the ring resonator radius, λ is the resonant wavelength in vacuum, and n
is the resonator refractive index. The result is an intense optical field in the ring resonator. Using the well-known ring resonator coupling analysis in [20
] and the fact that in the experiments presented here we typically observe 10% coupling from taper to LCORR (i.e. the dip shown in is 10% of the maximum), it can be shown that the power in the ring resonator is approximately 25 times that in the taper. Also, if critical coupling can be achieved, the power in the ring resonator with a Q-factor of 106
is as much as 250 times that of the taper.
(A) Experimental setup for measuring the Raman scattering signal from the LCORR. (B) Snapshot showing the LCORR capillary, the fiber taper, and the fiber probe.
As described earlier, however, it is the evanescent field at the inner surface that interacts with the sample. For the LCORR, the evanescent field is typically about 1% of the total power in the ring resonator. Although the power in the evanescent field is therefore similar to the power in the excitation taper, for Raman spectroscopy it is the intensity of the field that matters. Because the field protrudes only 100 nm into the sample, a high intensity can be achieved, particularly when a high degree of longitudinal confinement exists. This is the reason behind the excellent results predicted and achieved with microsphere-based SERS [15
In addition to orbiting the circumference of the LCORR, the high-Q resonant modes also propagate a short distance along the longitudinal axis. In fact, in , radiatively scattered light can be seen along the top of the LCORR. As a result, the Raman signal is generated across a longitudinal distance of approximately 1 mm. To collect the Raman scattered light, a polished multimode fiber is placed adjacent to the LCORR at the region of excitation. The location and angle are optimized to reduce the collection of radiative scattering from the WGM while not compromising the collection of Raman scattered photons. The probe fiber directs the photons to the spectrometer, where they first pass through a collimating lens, a high-pass Raman filter, and a focusing lens. A Triax 550 (Jobin Yvon) spectrometer with a resolution of 0.15 nm is used to analyze the Raman signal.
To characterize the Raman scattering signal from the sample to measure the average Raman enhancement due to the silver colloid, we use the experimental setup illustrated in , which is easily comparable with the LCORR experimental setup. An SMF-28 fiber cable is glued into a cuvette to deliver excitation light to the sample. The same probe fiber used in the LCORR measurements is placed just above the beam of light coming from the excitation fiber. shows the Raman signal measured with 33 nM R6G in the clustered silver colloid. Spectral lines are clearly visible at 1310 cm−1, 1360 cm−1, and 1510 cm−1, which are characteristic of R6G. show the peak intensity in the 1360 cm−1 line versus R6G concentration for cases with and without silver colloid, respectively. For this particular sample, the SERS enhancement is approximately 3 × 105, according to a comparison using the 1360 cm−1 line. Typical values of enhancement observed during these experiments vary from around 3 × 105 up to 106.
Fig. 4 (A) Experimental setup to characterize the SERS enhancement of the Ag colloid. (B) Measured Raman spectrum for 33 nM R6G in Ag colloid using the setup in (A). (C) Raman intensity versus R6G concentration in Ag colloid using the setup in (A). Each data (more ...)