1. Introduction

Whispering gallery mode (WGM) microresonators, featuring ultrahigh quality (Q) factors and small mode volumes, significantly enhance light-matter interactions [1]. Thus they are excellent platforms for bio/chemical sensors [2–4] by monitoring the resonant frequency shift [5–8] or the scattering-induced mode splitting [9,10]/broadening [11]. So far, optical WGM resonator based biosensors are mostly fabricated with silica [2], which is not biodegradable and therefore limit its applications for in-vivo biosening. In fact, biodegradation and biocompatibility are two of the most important factors for in-vivo biosensing [12]. In this case, a WGM microresonator made from biomaterial possessing both biodegradation and biocompatibility will be an excellent candidate for in-vivo biosensing [13]. Silk fibroin extracted from Bombyx mori (i.e., silkworm), has desirable properties for biosensing applications, including non-toxicity, biodegradability, biocompatibility, and high transparency in the visible light band (>95%) [14–20]. These excellent properties make it a suitable alternative material for WGM resonators.

In this paper, we report the fabrication and characterization of all-silk fibroin microtoroid resonators with Q factor on the order of 10⁵, dominated by material absorption loss. Furthermore, we have demonstrated thermal sensing based on silk fibroin microtoroid with the sensitivity as high as −1.17 nm/K, which is about 8 times higher than previous thermal sensors based on silica-based WGM resonators [21–26]. The high sensitivity of the silk resonator based thermal sensor originates from the large thermal expansion coefficient [27], which is three order of magnitude larger than that of silica.

2. Fabrication of the silk fibroin toroids

The fabrication process of silk fibroin microtoroid resonators is illustrated in Fig. 1(a). Specifically, regenerated silk water solution was extracted from Bombyx mori silkworm cocoons as described in Ref [18]. First, silk cocoons were cut into pieces and boiled in 0.02M Na₂CO₃ solution for 30 minutes. Subsequently, the silk fibroin was dried overnight in a fume hood. Then the dried silk was dissolved in 9.3 M LiBr solution for 4 hours at 60 °C, and the solution was dialyzed against ultra-pure water at room temperature for 48 hours. Next, 6 wt% silk solution was obtained by removing the small impurities with centrifugation. Meanwhile, Ultrahigh Q SiO₂ microtoroids were fabricated via standard processes described in Ref [28], and then used to generate molds for the silk resonators [29,30]. As a typical molding material of negative phase mold, polydimethylsiloxane (PDMS) was poured onto microtoroids that were silanized with trichloromethysilane, and then cured overnight at room temperature. Next, the resulting PDMS mold released from the microtoroid master was filled with regenerated silk fibroin solution to form silk based microresonators after being released from microtoroid master. Afterwards, the sample was placed inside the fume hood overnight at room temperature to dry the silk solution. Finally, the silk resonators were peeled off from the mold. Note that during this process the diameter of the resonator tended to shrink a little. Side-view and zoom-in top-view of the scanning electron microscope (SEM) images of a silk fibroin microtoroid are shown in Fig. 1(b), where an ultra-smooth surface can be seen.

 

Fig. 1 (a) Flow diagram illustrating the fabrication process of the silk toroids. 1) Bombyx mori Silkworm cocoons. 2) Regenerated silk fibroin solution was extracted from silk cocoon. 3) On-chip microtoroid resonator array was fabricated on a silicon wafer. 4) A negative phase PMDS mold mask was made from ultra-high-Q microtoroids on a silicon chip. 5) Transparent film with silk fibroin microresonator was obtained by dry-casting silk solution in the PDMS mold. 6) Optical image of a silk microtoroid resonator with diameter of 80 μm. Scale bar: (1) 5 cm, (2) 2 cm, (3)-(5) 5 mm, and (6) 40 μm. (b) Side-view and zoom-in top-view of the scanning electron microscopy (SEM) image of a silk fibroin microtoroid with diameter of 80 μm. Scale bar: 50 μm (sideview) and 5 μm (topview). The silk microresonator was coated with gold to obtain a better SEM image.

Download Full Size | PDF

3. Characterization of optical properties of silk microtoroids

To characterize the optical properties of the silk fibroin microtoroids, a tunable diode laser in 1450 nm wavelength band was used to excite WGMs in a silk fibroin microtoroid. We utilized a fiber taper with diameter of around 1.3 μm was utilized to couple the laser light into and out of the microresonators, as shown in Fig. 2(a). The transmitted light was then detected by a photoreceiver, and finally monitored by an oscilloscope. A typical transmission spectrum of a silk fibroin microtoroid with diameter about 100 μm is shown in Fig. 2(b), where a Q factor of 0.9 × 10⁵ was found by Lorenzian fitting of the spectrum lineshape (red curve) in 1450 nm wavelength band. The free spectral range (FSR ~4.06 nm) of the resonator, as shown in Fig. 2(c), can be observed by scanning the laser wavelength over a large range (1431 nm – 1439 nm), which agrees well with the measured resonator diameter (d ~104 μm from SEM measurement).

 

Fig. 2 (a) Illustration of the setup for testing and measuring the transmission spectrum of the silk fibroin microresonators. (b) Normalized transmission spectrum (blue curve) and the corresponding Lorentzian fitting (red curve) of a WGM in the silk fibroin microtoroid. (c) Wide-range transmission spectrum showing the free spectral range of the silk fibroin microtoroid.

Download Full Size | PDF

Furthermore, we have also measured the Q factors in different wavelength bands by utilizing different tunable lasers in 670 nm, 770 nm, 980 nm and 1450 nm bands. The Q factor in all these wavelength bands are on the order of 10⁵, specifically, with Q factor of 2.3 × 10⁵ in 670 nm band, 1.9 × 10⁵ in 770 nm band, 2.4 × 10⁵ in 970 nm band, and 0.9 × 10⁵ in 1450 nm band. In general, the intrinsic Q factor of a WGM microresonator is dominated by three kinds of losses: radiation loss, scattering loss and material absoption loss. The total Q factor can be expressed as $Q_{total}^{-1}=Q_{rad}^{-1}+Q_{scat}^{-1}+Q_{material}^{-1}$. The radiation loss dominated Q_rad is determined by the probe laser wavelength, the cavity diameter (100 μm), and the refractive index of the cavity material (1.55); thus Q_rad > 10¹⁰ can be easily obtained in this case [31]. On the other hand, the scattering loss induced Q_scat is dominated by the average roughness (R_a) of the resonator’s exterior surface, which is 1.849 nm for the silk toroid, confirmed by an atomic force microscopy (AFM) measurement (inset of Fig. 3). This implies a low scattering loss in silk microtoroid resonators, and thus Q_scat > 10⁷ is expected [32]. It should be noted that the long-range fluctuation of the toroid exterior surface is attributed to the curved surface of the toroidal structure [28]. Ultimately, silk’s material absoption loss is the main factor limiting the Q factor of the silk fibroin microtoroids in all the probe wavelength bands.

 

Fig. 3 Experimental Q factor of silk microtoroidal resonator in different wavelength bands. Inset: Atomic force microscopy (AFM) measurement of toroid’s exterior surface, with scanning area: 1 μm × 1 μm and R_a = 1.849 nm, showing that the surface roughness is small.

Download Full Size | PDF

4. Thermal sensor based on silk fibroin microtoroids

We have performed a thermal sensing experiment based on the fabricated silk fibroin microtoroids, as shown in Fig. 4. In the experiment, we placed a thermoelectric cooler (TEC) under the silk fibroin microtoroids to quantitatively change the local temperature by adjusting the driving current, and recorded the shift in resonant wavelength of a specific WGM at 1434.4 nm. Figure 4 clearly shows a sensitivity of -0.72 nm/K (triangle points). In the experiment, silk fibroin was processed from water solution, thus silk microtoroid was dominated by Silk I (random coil and α-helix conformation) [33]. When well-oriented β-sheet crystalline conformation is dominant in the device, the structure is in the.Silk II conformation, which is the general state of natural silk fibers. Since Silk II has a more crystalline regime, it has a greater sensitive thermal response [34]. We then used methanol to effectively crosslink the device and render it water-insoluble, thus leading to the transition from a Silk I to Silk II structure by inducing more hydrogen bonding within the material. Specifically, methanol vapor was generated in a glass dish on a hot plate at 40 °C. Then the silk toroidal resonators were suspended above the glass dish and exposed to methanol vapor for 4 minutes. The whole silk film was dried completely inside a fume hood at room temperature for 20 minutes after exposure was finished. The resulting device has a thermal response with a measured sensitivity of −1.17 nm/K (circle points in Fig. 4), which is about 8 times greater than what was previously reported for WGM thermal sensor [21–26].

 

Fig. 4 Resonant wavelength shift versus the temperature change for silk microtoroids with (circle) and without (triangle) methanol (MeOH) treatment. Silk microtoroid donminated by Silk I displayed a sensitity of −0.72 nm/K, while methanol treated silk resonator showed a higher sensitivity of −1.17 nm/K.

Download Full Size | PDF

The resonant wavelength shift of WGM versus temperature can be theoretically analyzed as follows [35,36],

5. Conclusions

In summary, we have demonstrated silk fibroin microtoroids on a flexible chip with Q factors as high as 10⁵, limited by the absorption loss. A thermal sensing experiment was also performed in this silk fibroin microtoroid which exhibited a measured sensitivity of -1.17 nm/K that is 8 times higher than the previous WGM resonator-based thermal sensors thanks to the large thermal expansion coefficient of silk. This work opens promising avenues to fabricate biodegradable and biocompatible protein-based microresonators for biophotonics applications that use natural materials as their constituents. This adds utility to the range of technical applications that are enabled by the use of structural proteins such as silk.