Au nanorod–coupled microfiber optical humidity sensors

: We demonstrate a high-sensitivity relative humidity (RH) sensor taking advantage of single-band narrow plasmon resonance of a single Au nanorod coupled to a whispering gallery cavity mode of a polyacrylamide microfiber. From the resonance peak shift, the sensor could achieve a sensitivity up to 0.51 nm/% RH with a cavity size of about 2 μ m. By coupling multiple Au nanorods along the microfiber axis, we demonstrate a position-dependent microfiber optical humidity sensor with a 1.5-mm spatial resolution, which can be potentially reduced to micrometer level, paving a way toward high-resolution distributed microfiber optical sensors.

depositing multiple Au NRs onto the microfiber to generate a series of distributed micrometer-scale hybrid cavities along the fiber axis, we demonstrated a prototype of distributed optical microfiber sensor with spatial resolution of 1.5 mm, which can be potentially reduced to the same level of the cavity size (i.e., ~2 μm).

Experiments
PAM microfibers were fabricated by direct drawing of a 2 wt% PAM (molecular weight: 5,000,000 to 6,000,000; Acros Organics) aqueous solution. Because PAM microfiber is water-soluble, in our experiment, Au NRs in aqueous solutions (NanoSeedz Ltd.) were firstly adhered onto a silica fiber taper, then Au NRs were transferred to the surface of the PAM microfiber by micromanipulation. Briefly, a fiber taper was firstly immersed into a dilute Au NRs aqueous solution for a few seconds and dried in the open air for ~2 hours, with Au NRs adhered onto the surface of the fiber taper. Then, under an optical microscope, the fiber taper mounted on a triple-axis micromanipulator was finely controlled to touch the surface of the PAM microfiber at an intersection angle of about 60° for several times, until single Au NRs were transferred onto the right position on the surface of the PAM microfiber.
To test the humidity sensing performance, the hybrid Au NRs/PAM structure was placed in an airtight poly(methyl methacrylate) chamber and the humidity was controlled by varying the ratio of dry and humidified nitrogen. The RH was detected by a commercial electronic hygrometer (EH) with its probe placed in the chamber. The resolution of the EH is 0.1%RH.
Optical properties of the hybrid Au NRs/PAM structure were characterized using a darkfield setup, as depicted in Fig. 1. A beam of unpolarized white light (SC-5, Wuhan Yangtze Soton Laser Co., Ltd.) from standard optical fiber was used to illuminate PAM microfiber coupled Au NR at an oblique angle of about 30° with respect to the microfiber axis, and a 50 × objective was used to collect the scattered light, which was then redirected to a chargecoupled device camera (DS -Fi2, Nikon) and a spectrometer (Maya2000-Pro, Ocean Optics). Using a dark-field microscope with a polarizer for imaging, it's easy to identify the orientation of a single Au NR from its scattering intensity, which presents a maximum when the selected polarization is along its longitudinal direction.
The morphology and size distribution of Au NRs were characterized by a transmission electron microscope (TEM, Hitachi HT7700), and the hybrid Au NR/PAM microfiber structure was characterized by a scanning electron microscope (SEM, Zeiss Utral 55).

Single-cavity-based optical RH sensor
We used a side-illuminated dark-field spectroscopy to investigate optical properties of single Au NRs coupled to a PAM microfiber. Figure 2 shows the scattering spectra of a single Au NR before and after coupling to the PAM microfiber. Before coupling, the longitudinal LSPR peak of an Au NR with length and diameter of about 110 nm and 38 nm deposited on a glass slide is about 700 nm with a linewidth of 46.5 nm ( Fig. 2(a)). After the Au NR was coupled to a 2.1-μm-diameter microfiber, the linewidth of the dominant LSPR peak reduced to 8.0 nm ( Fig. 2(b)). Here the linewidth is larger than that of hybrid Au NR and silica microfiber structure [22,23] due to the relatively larger optical loss of the polymer microfiber and the deviation of the NR orientation from vertical direction with respect to the microfiber length. Then the hybrid Au NR/PAM structure was placed in an airtight chamber with controllable humidity inside. When RH in the chamber is increased from 14.7 to 85.8%, the LSPR scattering spectra show a monotonous redshift as shown in Fig. 3(a). With RH increasing, more water molecules diffuse into the polymer microfiber, resulting in two competing factors: increased diameter of the microfiber (i.e., the WGM cavity) that leads to a redshift of the LSPR peak, and decreased refractive index of the microfiber that leads to a blueshift of the LSPR peak. Here the former factor overcompensates the later, resulting in a net redshift [10,[25][26][27]. The dependence of the LSPR scattering peak wavelength on RH shows excellent linearity for both increasing and decreasing RH over a wide RH range ( Fig. 3(b)), from which we obtain a sensitivity of about 0.51 nm/% RH. While this sensitivity is comparable with other high-sensitivity microfiber RH sensors [10,13,16,[18][19][20][28][29][30][31], the hybrid nanorodmicrofiber sensor is much more compact in size (i.e., the 2-μm cavity size of this sensor versus > 40-μm cavity sizes of other sensors) [9][10][11][12][16][17][18][19][20][21]. The miniature cavity size is also beneficial to fast response, as has been shown in other types of microfiber optical sensors [10,25,32].

Multiple-cavity-based distributed optical RH sensor
One special advantage of the hybrid nanorod-microfiber cavity is that, under appropriate illumination, wherever the nanorod deposited on the surface of the microfiber, it can locally excite a coupled LSPR and WGM mode. Since the coupled mode is confined within the micrometer-size cavity, when multiple nanorods are deposited along the fiber length, it is possible to generate a number of independent cavities for distributed optical sensing, as schematically illustrated in Fig. 4(a). To show this, we deposited three nanorods along a 2.1 μm-diameter microfiber, with 1.5 mm apart from each other. When a narrow stream of water vapor is blown onto the deposited area of the microfiber (Fig. 4(a)), a RH gradient along the microfiber length forms and the RH decreases successively from position P1 to P3, which is proved by the RH measurements using an EH (the open circles in Fig. 4(c)). LSPR scattering from the three nanorods, as shown in Fig. 4(b), gives different spectral shifts, clearly showing the RH gradient along the microfiber length, and confirming the possibility of developing distributed microfiber optical sensors with spatial resolution much higher than conventional fiber-optic sensors [42]. Normalized scattering spectra of 3 hybrid cavities at three different positions (denoted as P1, P2 and P3) before (RH = 57.7%) and after being exposed to water vapor. The insets are the corresponding dark-field microscopy images after being exposed to water vapor. The scale bars are 2 μm. (c) Position-dependent RH value measured by an electronic hygrometer (EH, circles) and this optical sensor (asterisks) before (balck) and after (red) being exposed to water vapor, respectively.
It should be noted that, as shown in Fig. 4(c), the RH distribution measured by the hybridcavity optical sensor, which is obtained with the sensitivity from Fig. 3(b) (i.e., 0.51 nm/% RH), is in excellent agreement with that by the EH. Limited by the diameter of the sensing chip of the EH (~0.8 mm), the separation between Au NRs was chosen to be 1.5 mm here. Since the spatial resolution of the hybrid nanorod-microfiber sensor is only limited by crosstalk between neighboring cavities (which may happen with the separation down to the cavity-size scale, e.g., 2 μm) and the optical diffraction limit (less than 2 μm in this work) for distinguishing scattering signals from neighboring nanorods in spectral measurement, this kind of distributed fiber-optic sensors in principle can offer a spatial resolution on micrometer level.

Conclusions
In conclusion, based on coupled WGM and LSPR modes of a hybrid Au NR/PAM microfiber cavity, we have demonstrated an optical humidity sensor with high sensitivity, high detection resolution, high spatial resolution, small footprint and simple structure. Moreover, by producing multiple independent micrometer-size cavities along the same microfiber in a scalable way, we have also demonstrated a prototype of distributed optical microfiber sensor with high spatial resolution. The sensor configuration can be extended for sensing many other measurands when the PAM microfiber or Au nanorod is pre-functionalized or replaced with counterparts made of other materials [43,44]. Finally, the initial results shown here may pave a way towards a category of coupled nanoparticle-microfiber structures for compact, high sensitivity and distributed photonic and plasmonic sensing.