TiO2-Coated Silica Photonic Crystal Capillaries for Plasmon-Free

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TiO2-Coated Silica Photonic Crystal Capillaries for Plasmon-Free SERS Analysis Bing Liu, Kan Wang, Bingbing Gao, Jie Lu, Haiming Li, and Xiang-Wei Zhao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00492 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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ACS Applied Nano Materials

TiO2-Coated Silica Photonic Crystal Capillaries for PlasmonFree SERS Analysis Bing Liu,†,‡,§ Kan Wang,†,‡ Bingbing Gao, ∥ Jie Lu, ⊥ Haiming Li,#,∇,○ and Xiangwei Zhao*†,‡,§ †State

Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, ‡National Demonstration Center for Experimental Biomedical Engineering Education, §Key Laboratory of Environmental Medicine Engineering of Ministry of Education, ⊥ School of Materials Science and Engineering, #State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210009, China ∥School of Pharmaceutical Sciences and School of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China. ∇School of Telecommunications and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing, 210003, China ○Key Laboratory of Radar Imaging and Microwave Photonics, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China

ABSTRACT: Rapid and simple development of point-of-care testing (POCT) device with convenience, easy operation, reusable, and high selectivity and sensitivity is remaining a challenge. Herein, we developed a simple and economical technique to fabricate a novel POCT sensor for plasmon-free surface enhanced Raman scattering (SERS) analysis. In practice, capillary and silica photonic crystal were used as support and framework, respectively, and then followed by atomic layer deposition (ALD) of titanium dioxide (TiO2) on the photonic crystal framework for the formation of shell structure. It was found that the sensor gained the enhancement factor (EF) of 3.63×104, and it exhibited a highly selective and sensitive detection for methylene blue (MB) with a good linearity in the range from 10-7 to 10-2 M (R2 = 0.997) and the detection limit of 72 nM, which is attributed to the enhanced matter-light interaction by whispering gallery mode (WGM) resonance and multiple light scattering with TiO2-coated photonic crystal capillary (TiO2-PCC), as well as chemical enhancement of TiO2. More importantly, the as-proposed sensor could be regenerated under simple irradiation of UV light owing to the photocatalytic property of TiO2. We anticipate this sensor to be widely used in the POCT filed of resource-constrained areas. KEYWORDS: SERS, titanium dioxide, photonic crystal, POCT, regeneration

INTRODUCTION With the rapid development of modern times, point-of-care testing (POCT) is attracting more and more attention among people who are at the risk of sub-health, which can be

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easily operated by an individual without the aid of laboratory equipment, staff, and venues.1 This is one of the major reasons that the paper-based POCT devices take leadership role in the human health monitoring.2 Generally, they employ colorimetric or fluorescence respectively by the aggregation of noble metal nanoparticles or fluorescent dyes for qualitative, semi-quantitative, or quantitative detection.3,4 However, the sensitivity of the colorimetric-based paper strip is insufficient, and the another type of strip usually suffers from the stability of optical signal due to its inherent disadvantages of photobleaching and quenching,5 which will restrict their further application in development of the personalized medical programs. In addition, the cost of fluorescently modified nanoparticles is very high, as well as the complexity of device fabrication and time consumption of experimental process. Also, these commercial paper-based POCT devices are not reusable. Consequently, in order to overcome the limitations of the existing analytical devices, many efforts are remained required to develop novel, simple, and economical fabrication methods for design of reusable POCT devices with high selectivity and sensitivity. Recently, surface enhanced Raman scattering (SERS) has become an important and powerful analytical technique, which has been widely used in various fields, such as imaging, biology and chemical analysis, and environmental monitoring.6-14 The reason is that SERS has many significant advantages compared with commonly used fluorescence. First, SERS has higher stability, which can resist photobleaching, quenching, and autofluorescence.15,16 Second, SERS exhibits narrow emission bands, which is desirable for multiplex detection.17,18 Third, SERS spectrum offers abundant fingerprint information of analyte or label molecules, as well as providing high sensitivity.19,20 How to fabricate SERS active substrates with strong enhancement effect seems to be a focus in SERS applications.14,21-23 Up to now, the mechanisms of SERS enhancement which have been accepted are electromagnetic and chemical enhancements,24 which attribute to the localized surface plasmon resonance (LSPR) effect and charge transfer, respectively. In general, some noble metal nanomaterials are used as the common enhanced agents based on LSPR, such as gold, silver, and copper nanoparticles.25 However, these noble metal-based SERS substrates usually suffer from high-loss of resonance energy, which is harmful for the electromagnetic enhancement of the substrate, resulting in a weak Raman signal.26 In addition, these standard SERS substrates are usually for one-time use only, and considering the cost of the noble metals, these SERS substrates cannot be thoroughly explored as conventional analytical techniques. Therefore, the research should be focus on the development of reusable SERS-active substrates. High refractive index material can effective overcome the aforementioned shortcomings.27 Therefore, the SERS substrate consisted of high refractive index materials can reduce the loss of resonance energy, which is valuable for high enhancement effect of SERS. Titanium dioxide (TiO2) is a broad investigated semiconductor material in the fields of environmental remediation and clean energy resources because of its photocatalytic characteristic, chemical stability, nontoxicity, and low cost.28-31 In addition, TiO2 with high refractive index, has been used in SERS applications based on charge transfer.27,32-


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39

When the target molecule is adsorbed on the surface of TiO2-based substrate, the TiO2 plays an extreme important role in the process of charge transfer, which is not only used as the intermediate state, but also the electron traps for the electron transfer to molecule, resulting in the magnification of the molecular polarizability, and subsequently the intensity of Raman signal is enhanced.40 In recent years, great progresses have been made in the research of SERS based on TiO2. For example, Alessandri prepared a TiO2 shell-based resonator.41 It was found that the resonator exhibits unprecedented SERS enhancement effect, which is achieved by the synergistic effect of the combination of high refractive index of the shell material and multiple light scattering, resulting in the generation of local surface “hotspots” by whispering gallery mode (WGM) resonance. Besides the resonator, Qi et al. developed a TiO2 inverse opal photonic crystal substrate, which has great SERS enhancement with plasmon-free mode owing to the strong interaction of matter-light through repeated and multiple light scattering in photonic microarray.27 It was also illustrated that the enhancement effect is determined by the different light coupling performance of microarray with multifarious photonic band gaps. When the band gap center of the microarray is closest to the wavelength of the excited light, the SERS enhancement of the microarray is low because of the suppression of the propagation of the light. On the other hand, while its band edges near to laser wavelength, the microarray produces slow light effect, resulting in higher enhancement effect. However, the construction process of these devices is complex and not suitable for POCT. Hence, in this paper, we developed a plasmon-free sensor composed of TiO2-coated silica photonic crystal capillary (TiO2-PCC) for SERS analysis. As demonstrated in Figure 1, we first used the capillary as carrier for the preparation of the silica photonic crystal. Then, by the mean of atomic layer deposition (ALD) method, TiO2 with different thicknesses were deposited on the inner surface of silica photonic crystal capillary (PCC) for producing WGM resonator, which could be used to adjust the band gap of the photonic crystal. ALD method is a self-limiting high-level coating film technique that allows the formation of low porosity on a sub-monolayer scale.42 Next, this sensor was directly applied to the detections of methylene blue (MB) and dopamine (DA) in label-free mode. Our results showed that the as-proposed sensor exhibited excellent SERS enhancement effect for the sensitive detection, as well as convenient and high stability because of the light modulation and filtering characteristics of PCC,43,44 which allows to delivery, separation, and analysis of samples simultaneously. More importantly, the sensor possesses reusability and wide line dynamic detection range (LDR) because of the photocatalytic of TiO2 and the porous photonic crystal capillary with high surface to volume ratio, respectively. In addition, the manufacturing costs were significantly reduced without the introduction of the noble metal, which holds great promise for the development of POCT device.



Figure 1. Schematic illustration of the fabrication of SERS substrate based on TiO2-PCC.

EXPERIMENTAL SECTION Materials. All water was distilled and purified to Milli-Q quality. Monodisperse colloidal silica nanoparticles were purchased from Nanjing Nanorainbow Biotechnology Co., Ltd. (China). Capillary was commercially available from Fiberguide Industries (USA). Acetone, sulfuric acid (H2SO4, 98 %), hydrogen peroxide (H2O2, 30 %), and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd (China). Methylene blue (MB) and dopamine were obtained from Sigma-Aldrich. Titanium tetraisopropoxide (97 %) was obtained from Alfa Aesar. Phosphate-buffered saline (PBS, 0.05 M, pH 7.4) was prepared in-house. Cleaning and hydrophilic modification of capillaries. In order to remove the polyimide coating of capillary, first, the capillary with inner and outer diameters of 180 μm and 360 μm was cut into small pieces of 5 cm long, and then these capillaries were calcinated at 500 °C for 3 h. Second, the capillaries were cleaned in ultrasonic cleaner for 5 min while immersed in acetone. Afterward, they were cleaned by ultrapure water and subsequently dried by nitrogen gas. Next, the capillaries were cleaned in ultrasonic cleaner for 30 min with using ethanol, and cleaned by ultrapure water and dried by nitrogen gas. Repeat the above process twice. Finally, the capillaries were immersed in piranha solution for hydrophilic modification for 12 h and dried by nitrogen gas after washing. Preparation of silica photonic crystal capillaries. The silica photonic crystal capillary was fabricated by self-assembly of monodispersed silica nanoparticles with the diameter of 275 nm in the clean capillary. Briefly, 5 % silica colloidal solution was infiltrated into the capillary by the capillary force. Next, it was dried at room temperature (25 °C) for 2 days. Finally, it was calcinated at 500 °C for 4 h to increase its mechanical strength.


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TiO2-coated silica photonic crystal capillary by ALD method. To obtain TiO2-PCC, first, capillary was exposed to Ti and O atom steam at 150 °C sequentially. Here, titanium tetraisopropoxide and ultrapure water were respectively used as Ti and O precursors, and they were kept at 70 °C and 30 °C, respectively. The pulse, exposure and purge times of Ti and O precursors all were 1, 8, and 20 s, respectively. Nitrogen was used as purge gas. Next, by controlling the number of deposition cycles, different thicknesses of TiO2 shell (1.8, 2.7, 3.6, 4.5, and 5.4 nm) were deposited on the inner surface of the photonic crystal capillaries, respectively, which were used for exploring the relationship between reflection peak position of photonic crystal and the thickness of TiO2 shell. Finally, the PCC with the TiO2 shell thickness of 10 nm was used for further experiments. SERS and recyclable evaluation of TiO2-coated photonic crystal capillaries. To evaluate SERS performance of the as-proposed PCC, MB solutions ranging from 10-7 to 10-2 M were used as an indicator for SERS measurement. First, MB solution was infiltrated into TiO2-PCC by capillary force, and dried at room temperature in a dark environment. Next, Raman signals of the MB into the capillaries were measured using a Raman spectrometer equipped with a microscope, and the wavelengths of the excited light were 532, 633, and 785 nm with a laser power of 5 mW, respectively, and the data acquisition time was set to 10 s. In order to demonstrate the recyclable property of the TiO2-PCC, the capillary loaded with MB of 10-3 M was immersed in the ultrapure water and then irradiated with a UV lamp (365 nm, 10 mW cm-1) at room temperature for 40 min according to our previous report.45 Then the capillary was washed with ultrapure water to remove residual ions and molecules, and subsequently dried in nitrogen flow. The rinsing process was repeated four times to make sure that the capillary became clean. After that, Raman spectrum was recorded by the Raman spectrometer. MB was resorbed into the capillary after self-cleaning, and the recycling process was repeated four times. SERS detection of dopamine in aqueous medium and human urine samples. To illustrate the dopamine detection with the as-proposed TiO2-PCC, different concentrations of DA from 10-6 to 10-1 M were prepared. Next, the capillaries were immersed in the above DA solutions for 3 min to absorb DA molecules into the capillaries, and also dried in room temperature. Finally, Raman signal of the capillary was recorded by Raman spectrometer with the laser wavelength of 785 nm and exposure time of 10 s. In order to test the ability of the capillary for real human urine samples detection, the fresh human urine that was received from healthy volunteers was firstly diluted by 100 times with PBS. Next, various amounts of dopamine were added to obtain the real urine samples, and the processes of sample absorption and Raman measurement of DA were the same as that in aqueous medium. Electromagnetic calculation. We used a commercial software (FDTD Solutions,



Lumerical Inc.) for electromagnetic calculation. As shown in Figure S-1 in the Supporting Information, the calculation model included a photonic crystal structure and a quartz glass substrate. The photonic crystal structure consisted of 12 layers of TiO2coated silica nanoparticles stacked along the z-direction and an array of 4×4 nanoparticles in the x-y plane, which was self-assembled in the form of Face-Centered Cubic (FCC). The thicknesses of the TiO2 shell and glass substrate were 10 nm and 1,000 nm, respectively, and the diameter of the silica nanoparticle was 275 nm. The boundary conditions of the computational domain were set to the periodic one in the horizontal direction (x-y plane) and the absorbing one in the z-direction. The refractive indices of silica, TiO2, and water were 1.45, 2.41, and 1.33, respectively, and the glass substrate was based on the parameter of SiO2 (Glass)-Palik. The excited lights with the wavelengths of 532, 633, and 785 nm incident along the z-axis were used for excitation, respectively. Instrumentation. A tube furnace (STF54434C, Lindberg Blue M, ThermoFisher, USA) was used to calcinate capillaries for acquiring silica photonic crystal templates. A scanning electron microscope (FESEM; Zeiss, Ultra Plus) was employed to characterize structure of the cross section of capillaries. The TiO2 thin films were deposited on the photonic crystal templates using a self-made atomic layer deposition system developed by the Institute of Coal Chemistry, Chinse Academy of Sciences (China). The SERS measurements of the capillaries were carried out by an InVia Renishaw Raman microscope (Renishaw, UK) equipped with 20× Leica objective lens. The reflection spectra of silica photonic crystal and TiO2-coated structure were recorded by an optical fiber spectrometer (Ocean Optics, QE65000) coupled to an optical microscope (Olympus, BX51) with a 50× objective lens. RESULTS AND DISCUSSION Characterization of TiO2-coated photonic crystal capillaries. In our strategy, the TiO2-PCC acts both support of target molecules and plasmon-free SERS enhancement substrate. Therefore, the TiO2 needs to be uniformly deposited on the surface of photonic crystal in the capillary, to generate stronger SERS enhancement with uniform distribution. In addition, the PCC must have higher mechanical strength and chemical stability, which is of certain value for the determination of reproducibility. Through sintering, the silica nanoparticles with the diameter of 275 nm were connected to each other for the formation of PCC with low fluorescence background.46 The capillary also had very low Raman background after TiO2 shell coating (Figure S-2a in the Supporting Information), which will not interfere with the original Raman signal of the target analyte, leading to the enhancement of signal-to-noise ratio (SNR) from the point of view of the support. As shown in Figure 2a, it could be seen that the silica photonic crystal was porous structure that was interconnected by monodispersed nanoparticles, and its surface displayed hexagonal alignment through self-assembly of silica nanoparticles. From the SEM image of the cross section of PCC as shown in the inset of Figure 2a, it could be found that the silica nanoparticles were uniform aligned with


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large area, which is desirable for the deposition of TiO2 on the surface of the photonic crystal. The SEM images of the photonic crystal capillaries with different thicknesses of TiO2 shell coating (10, 20, and 30 nm) also demonstrated the uniformity of the photonic crystals in the capillaries (Figure 2b-d). The reason is that ALD method, as a high-level coating technology, can accurately control the thickness of the deposited layer. From SEM images as shown in Figure 2b-d, it could be seen that the gap between nanoparticles gradually decreased after the increase of the TiO2 shell thickness. In addition, the cross profile of the nanoparticles transformed from circular to regular hexagonal. Therefore, through this means, the TiO2 shell with different thicknesses coated photonic crystal capillaries could be used to prepare uniform detection platform, which is conducive to improving the robustness and reproducibility of detection.

Figure 2. SEM images of TiO2-PCC. (a) SEM image of the surface of silica photonic crystal. Inset: SEM image of the cross section of capillary. SEM images of the cross section of photonic crystal capillaries coated with different thicknesses of TiO2, (b) 10 nm, (c) 20 nm, (d) 30 nm. The scale bar of the inset is 50 μm.

Since SERS enhancement on the basis of the coupling between the wavelength of excited laser and the band gap of the PCC,47 we first explored the relationship between the thickness of the TiO2 shell and the reflection peak position of the PCC. According to Jeffrey’s work, the reflection peak position of the PCC will red-shift with the increase of the TiO2 shell thickness.48 Then, we carried out some approximate calculations that as follows. Under normal incidence, the reflection peak position of a photonic crystal capillary could be estimated by the law49  =1.633dnaverage

(1)

where  represents the peak wavelength, d stands for the center-to-center distance between two neighboring nanoparticles, and naverage is the average refractive index. The



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naverage was calculated by the following equation

naverage  SiO2 nSiO2 2  TiO2 nTiO2 2  air nair 2

(2)

where nSiO2 , nTiO2 , and nair represent the refractive indices of silica, TiO2, and air, respectively, and their values are nSiO2  1.47 , nTiO2  2.50 , and nair  1.00 , respectively.

SiO2 , TiO2 , and air are the volume fractions of silica, TiO2, and air, respectively, and

SiO  74 % , air  26 %  TiO2 . Therefore, the volume of TiO2 shell was determined 2

by the following formula VTiO2

3    d    1  VSiO  1  2  D   2  

(3)

where d represents the thickness of TiO2 shell, and D stands for the diameter of silica nanoparticle. The volume fractions of the TiO2 shell and silica nanoparticle could be estimated by the following formula

TiO

2

3    d     1  1  SiO2  D   2  

(4)

The theoretical values of the reflection peaks of the PCCs with different thicknesses of the TiO2 shells coating are illustrated in Table 1. It could be seen that the reflection peak position of the PCC red-shifted about 12 nm with the increase of the thickness of TiO2 shell. However, the actual measured values were lower than the corresponding theoretical values, which were 610, 622, 633, 635, 638, and 642 nm, respectively (Figure 3a and b). Compared with the theoretical value of capillary without TiO2 shell coating, the actual measured value slightly blue-shifted. The reason is that the lattice constant of the silica photonic crystal is decreased after calcination. On the other hand, for the capillary with various thicknesses of TiO2 shell coating, the measured values showed a larger blue-shift. In addition, the thicker TiO2 shell coating in capillary, the greater the amplitude of the blue-shift. This is owing to the computational errors that the ideal model we have built. Actually, silica nanoparticles are cross-linked each other after calcination, the volume fraction of TiO2 shell changes smaller compare to the theoretical value which we used for the calculation of the naverage value, and the error changes greater with the thicker TiO2 shell. Moreover, the environmental conditions and sample defects perhaps result in errors during the preparation and characteristic of the TiO2-PCC. From the optical images of the capillaries coated with different thicknesses of TiO2 shell as shown in Figure 3c-h, it was clear that the colors of the


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photonic crystal capillaries were changed from orange to red, which were consistent with the reflection peak positions. This also proves the accuracy of our calculation. Table 1. The theoretical values of the reflection peak of the PCCs with different thicknesses of the TiO2 shells coating.

TiO2 thickness (nm)

naverage

 (nm)

0

1.363

612

1.8

1.418

637

2.7

1.446

649

3.6

1.474

662

4.5

1.501

674

5.4

1.528

686

Figure 3. (a) Reflection spectra of the TiO2-PCCs coated with different thicknesses of the TiO2 shell. (b) Comparison of the theoretical and the actual measured values of the TiO2-PCCs. (c)-(h) Optical images of the TiO2-PCCs. (c) 0 nm, (d) 1.8 nm, (e) 2.7 nm, (f) 3.6 nm, (g) 4.5 nm, (h) 5.4 nm. All scale bars are 100 μm.

Evaluation of TiO2-coated photonic crystal capillaries. In order to evaluate the enhancement performance of the as-proposed SERS substrate, a quantitative assay was performed for MB label-free detection. Here, we used the PCCs coated with 10 nm thick TiO2 shell as the carriers. The photonic crystal was composed of 275 nm silica



nanoparticles connected to each other by sintering, whose pore size was less than 41 nm (3/20 of the nanoparticle diameter). The pore size became smaller after the TiO2 coating, which is harmful for the flow of analyte liquid. Therefore, we extended the infiltration time of sample to 3 min, which also achieved the purpose of separation and purification. The SERS enhancement effect of analyte in the capillary is regulated by band edge of the photonic crystal.26 Hence, we first selected the laser that its wavelength coupled to the band edge. MB, as a common indicator of SERS performance, several obvious characteristic Raman shifts have been recorded, which are at 481 cm-1 (C-S-C symmetrical stretching), 1435 cm-1 (C-N stretching), and 1627 cm-1 (C-C stretching).50 Here, we employed most commonly used three wavelengths of laser as the excited sources, which were 523, 633, and 785 nm, respectively. The result shown in Figure 4a indicated that the intensity of Raman shift at 1627 cm-1 of MB was the strongest with single molecule signal intensity of 3.96×10-13 at the concentration of 10-3 M under the excitation of 785 nm laser. The Raman signal intensities with 532 and 633 nm excitations were almost equal. However, the average Raman signal intensity of a single MB molecule at 532 nm laser (3.22×10-13) is higher than that of 633 nm laser (8.6×1014) due to the difference in the spot size between 532 and 633 nm lasers. This significant Raman enhancement can be explained by the synergistic contribution of the following three main factors. First, the higher refractive index contrast (2.50 for TiO2, 1.47 for SiO2) can cause total internal reflection of light in the TiO2 shell. Second, the TiO2PCC is serviced as extended Fabry-Pérot cavity, resulting in multiple light scattering when the wavelength of excitation is coupled with the band edge of the PCC because of the slow light effect (Figure S-2b in the Supporting Information).27,41 Third, this enhancement is also attributed to the charge transfer mechanism of TiO2. In order to understand the mechanism of the excited source and the TiO2-PCC, we performed an electromagnetic simulation with FDTD software for calculating the local field distributions of the silica and TiO2-coated photonic crystal capillaries, respectively. Under the irradiation of 532, 633, and 785 nm lasers, the electric field distributions of the silica photonic crystal capillary are respectively shown in Figure S-3a-c in the Supporting Information. It could be seen that the electromagnetic fields were redistributed inside the capillaries under different excitations, and the highest electric field was gained with illuminating at 633 nm (|E/Einc|=3.31), while the near field profile of |E/Einc| with 785 nm laser was about 3.24. In addition, the simulation results of the TiO2-PCC demonstrated that the minimum electromagnetic intensity was obtained with 785 nm excitation (|E/Einc|=3.4) (Figure S-3d-f in the Supporting Information). However, the energy hotspots were uniformly distributed, which is advantageous for the reproducibility and reliability of detection. On the other hand, the electromagnetic fields had similar strengths under excitation of 532 and 633 nm lasers, and their enhanced electric fields were randomly distributed. This is contrary to the SERS performances observed in the experiment. The possible reason is that SERS mainly originated from chemical enhancement on the surface of semiconductor material, while


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the WGM resonance and multiple light scattering mode of the TiO2-PCC have little effect. And it can be explained by charge transfer mode as follows. Electrons of TiO2 valance band (VB) are excited to surface state energy levels by the 785 nm laser with the sub-band gap energy and then injected into the lowest unoccupied molecular orbital (LUMO) of target molecules.51 Furthermore, the results also show that the WGM resonance has slight effect on SERS enhancement because of the only extra 0.16 times enhancement factor. Therefore, the 785 nm laser was chosen as the excited source for producing an excellent signal-to-noise ratio in the following analysis. As a proof-of-concept experiment, MB solutions with concentrations ranging from 10-7 to 10-2 M were employed to the evaluation of quantitative performance of the SERS substrate. The results of the intensities of Raman shift of the photonic crystal at 1627 cm-1 are plotted against the concentration of MB, as shown in Figure 4b and c. It indicated that the linear dynamic detection range was from 10-7 to 10-2 M with the R2 value of 0.997 (Figure 4c, inset), spanning 5 orders of magnitude, which is much wider than that of other detection method.52 The limit of detection (LOD) was evaluated to be 72 nM at the SNR of 3:1, which is also much lower than that reported in previous study.52 This enhanced sensitivity can be associated with charge transfer generated at the interface of TiO2 and MB. In addition, the extremely low Raman background and good signal-to-noise are also attributed to the increased sensitivity. Although the sensitivity is still lower than those of the conventional noble metal-based SERS substrates, the photonic crystal capillary can avoid the interference of other macromolecular substances because of its properties of delivery, separation, and analysis of samples simultaneously, which opens a new avenue for the application of POCT. Most importantly, these processes do not require the introduction of any metal enhancers, which can avoid interference, as well as decrease the fabricating cost of device.

Figure 4. (a) Raman spectra of MB label-free detection with the TiO2-PCC at excitation of 532, 633, and 785 nm



lasers when the MB concentration is 10-3 M. (b) Raman spectra of MB with different concentrations ranging from 10-7 to 10-2 M. (c) Reference plot of intensities of Raman shift at 1627 cm-1 vs MB concentration. Inset is the linear exhibition of the reference plot. Error bar is obtained from five repeats. (d) Optical image of the PCC, and the seven back dots present the position of detection spots. (e) Raman spectra of seven detection spots. (f) Statistical analysis of signal intensities of Raman shift at 1627 cm-1 of MB.

Besides high sensitivity and wide LDR, uniformity, reproducibility, and stability are of great importance when considering the practical application of SERS platform. First, the as-proposed SERS substrate provides fingerprinting molecular vibrational spectrum of analyte, which is intrinsically much more stable without bleaching and quenching in comparison with fluorescent materials. Therefore, it is great promising and powerful for improving detection robustness and reproducibility. Second, in order to estimate the uniformity of a single capillary, we chose seven points according to the principle of symmetry and recorded their SERS spectra (Figure 4d). As shown in Figure 4e and f, the intensity of Raman shift at 1627 cm-1 were measured, and the coefficient of variation was calculated to be 4.2 %. In addition, the relative standard deviation of the detection among different batches was as low as 6.3 % (Figure S-4 in the Supporting Information), indicating the advantages of the fabricated detection platform as aspect to good uniformity, high reproducibility and stability, as well as low background signals. All of these good performances endorse the use of the TiO2-PCC as a powerful tool for rapid, sensitive, and label-free SERS detection. Recyclability of TiO2-coated photonic crystal capillaries. Owing to the intrinsic photodegradation activity of TiO2, the TiO2-PCC can decompose adsorbed molecules under the irradiation of UV light, which makes sense for routine applications.53,54 Herein, a higher concentration of MB with 10-3 M was loaded into the capillary for photocatalytic bleaching test under UV illumination. Figure 5a and b provide Raman spectra of MB before and after 40 min of exposure to UV light, and four recycles of the detection-cleaning process were carried out. The result clearly showed that all Raman shift peaks of MB had almost vanished after the first recycle, illustrating that MB is completely photodegraded in a rapid and effective manner. The reason may be attribute to the high light harvesting capacity of substrate. Since the target molecules are degraded to inorganic substances, including carbon dioxide, hydrochloric acid, and water, which can be easily removed by aqueous solvent,31 the assay practice is simplified by just four times washing. Subsequently, the capillary was continuously impregnated in the above MB solution for the second recycle. The measured Raman spectra showed that Raman shift intensity at 1627 cm-1 was similar to that of previously recycled MB, which also proved that the degradation of MB in the capillary was achieved successfully (Figure 5b). In addition, after four recycles, the characteristic Raman signals recorded on the cleaned substrate disappeared completely, and the Raman spectra were similar to that of a new PCC. What is more, the results showed that the intensities of the identified peak of MB remained at almost the same level in each detection step, indicating its excellent SERS activity after four runs of circulation.


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Therefore, by this manner, the TiO2-PCC could endure multiple UV illuminations, enabling it to serve as a feasible recyclable SERS sensor with high robustness, stability, and reproducibility.

Figure 5. (a) Recycled test of the TiO2-PCC in the presence of 10-3 M MB. (b) The variation is Raman intensity of the TiO2-PCC after UV-illumination cycles. (c) Raman spectra of different concentrations of DA (from 10-6 to 10-1 M). (d) Reference plot of Raman intensities of Raman shift at 783 cm-1 for varying concentrations of DA. The inset is the linear calibration curve. (e) Raman spectra of real human urine samples. (f) Correlation between the performance of the as-proposed method (solid squares) and reference concentrations (hollow squares) for DA. Error bar is calculated from five repeats.

Dopamine analysis. To further confirm the sensor performance based on the TiO2PCC, we determined a series of solutions including various amounts of dopamine (the concentrations range from 10-6 to 10-1 M). DA, as an important neurotransmitter, is essential for functions of human brain and body, which becomes one of most studied biomolecules because of its indicative role in physiological and psychological processes.55-57 From the SERS spectra, as shown in Figure 5c, it could be clearly seen that the intensity of Raman band at 783 cm-1 was gradually increased by enlarging the addition of dopamine. The Raman band intensities are plotted against the concentration of DA are listed in Figure 5d, and the inset shows the logarithmic concentration of linear calibration curve. These results illustrated the corresponding determination range was 10-6 to 10-1 M, and the LOD at the SNR of 3:1 was calculated to be 550 nM, which is lower than that of other methods reported in previous studies (Table S-1 in the Supporting Information).58,59 In addition, the as-proposed SERS device exhibited good linearity in the range from 10-6 to 10-1 M with a correlation coefficient of 0.995, enlarging the LDR to 5 orders of magnitude. which is also wider than that of the other assay reported (Table S-1 in the Supporting Information).58,59 The desirable detection property can be resulted from the advantages mentioned above. Also, the high surface to volume ratio of the PCC probably attributes to extend the detection range. It should



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be noted that such the LOD is higher than that of the noble nanostructures-based optical and electrochemical sensing methods (Table S-1 in the Supporting Information).60-62 However, the LDR of the as-proposed sensor is wider, which avoids the pretreatment of samples.61,62 Therefore, the analysis instrument based on the PCC can realize the requirement of the next acquisition technique with high throughput screening and sequencing in the post-genomics era, which is of great significance for precision medicine. Subsequently, in order to investigate the enhancement factor (EF) of the TiO2-PCC, we evaluated the SERS performances of the bare and TiO2-PCCs, respectively, and the calculation was carried out according to the following equation27 EF=( I SERS / N SERS ) / ( I Bare / N Bare )=

where

N SERS

and

N Bare

I SERS N Bare I Bare N SERS

(5)

present the average number of DA molecules in scattering area

measured by SERS and no-SERS measurement for the TiO2-coated and bare photonic crystal capillaries, respectively.

I SERS

and

I Bare

stand for the intensities of the

vibrational modes in the SERS spectra and normal Raman spectra. In this research, we employed the DA concentration of 10-2 M for evaluating EF. Here, we assumed that the target molecules of the measurement area were evenly distributed, the numbers of molecules were estimated by equation27 (6)

N  CVN A S scan / S sub

where

NA

is Avogadro constant, C and V stand for the molar concentration and

volume of the analyte solution in the scattering area, respectively.

S scan

and

S sub

present the areas of illumination scanning and the substrate, respectively. Raman spectra of DA absorbed into the bare capillary and TiO2-PCC are shown in Figure S-5 in the Supporting Information. The EF at the main characteristic Raman band of 783 cm-1 of DA was calculated to be about 3.63×104, providing direct evidence to the high enhancement for the plasmon-free TiO2-PCC, which is beneficial for reliable DA detection. It is worth noting that the WGM resonance and multiple light scattering of the TiO2-PCC offer an enhancement factor of about 133.63, which highlights the role of chemical enhancement in the structure. SERS detection of dopamine in human urine samples. To demonstrate the practical feasibility of the as-proposed SERS strategy, we designed an experiment for DA detection in healthy human urine, as a model of complicated matrix, including a lot of water, urea, uric acid, and inorganic salt. First, the certain amounts of DA were added to the healthy urine samples for demonstrating the applicability and reliably of this sensor, and the DA concentrations of the five urine samples were 6.7 μM, 73.6 μM,


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514.3 μM, 3.5 mM, and 62.3 mM, respectively. As shown in Figure 5e and f, the results acquired by the as-developed sensor showed good agreement with the standard addition as the correlation coefficient for DA detection is 0.998, and the recovery rates of DA were determined to be 96.25 %-104.13 % (Table S-2 in the Supporting Information), respectively, which illustrate that the accuracy of the present strategy for DA detection in real samples. Besides, all of these relative standard deviations (RSD) were below 5.46 % in the five repeated parallel tests, proving the reproducibility of the SERS substrate. These results strongly confirm the great application potential of the SERS platform in clinical analysis. CONCLUSIONS In conclusion, we have developed a convenient and economical method for the preparation of a novel POCT device based on TiO2-PCC by label-free manner. The thickness of TiO2 deposited on the inner surface of the PCC could be precise controlled by the ALD method. The results indicated that the reflection band of the SERS substrate was evidently red-shift with the increase of the thickness of TiO2 shell, which is used for regulating the band edge of photonic crystal to match the wavelength of the excited source, leading to the enhancement of Raman signal for sensitive detection. Dopamine analysis demonstrated that the as-proposed sensor exhibited excellent analytical performance with detection limit of 550 nM and the wide LDR ranging from 10-6 to 10-1 M, which is better than that of electrochemical sensing methods. In addition, this SERS substrate provided high enhancement factor of about 3.63×104 on the basis of its chemical enhancement, WGM resonance, and multiple light scattering. Meantime, the sensor displayed extremely low background signals in real human urine samples, as well as high reproducibility, stability, and reliability. More importantly, the as-proposed SERS platform could be reused after simple UV irradiation, which is valuable for the application in limited resource setting. All of these characteristics meet the requirements of novel SERS substrate for medical and biological analysis, and provide potential applications in the design of POCT device. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: FDTD simulation model, Raman, and reflection spectra of the TiO2-PCC; Calculated near field distributions of the silica and TiO2-PCCs at the different resonant wavelengths; Reproducibility test of five batches of the TiO2-PCCs; Raman spectra of dopamine adsorbed into the bare and TiO2-PCCs; Comparison of dopamine detection between different methods; Detection and recovery test of dopamine in the human urine samples (PDF) AUTHOR INFORMATION



Corresponding Authors *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2018YFF0215200, No. 2017YFA0205700), National Natural Science Foundation of China (Grants 61850410528, 81827901, 21327902 and 61701253), the Natural Science Foundation of Jiangsu Province (No. BK2014021828, BE2016002 and BK20170907), China Postdoctoral Science Foundation funded project (2018M642132, 2018T110428 and 2017M621597), Fundamental Research Funds for the Central Universities, Six Talent Peaks Project of Jiangsu Province, the Collaboration Research Fund of Southeast University and Nanjing Medical University (Grant No. 2242017K3DN26), Fundamental Research Project of Shenzhen Science & Technology innovation Committee (201803063001075), Open Research Program in China’s State Key Laboratory of Millimeter Waves (K201809), Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Sponsored by NUPTSF (NY217122). REFERENCES (1) Li, Z.; Liu, H.; He, X.; Xu, F.; Li, F. Pen-On-Paper Strategies for Point-of-Care Testing of Human Health. Trac-Trend. Anal. Chem. 2018, 108, 50-64. (2) Lepowsky, E.; Ghaderinezhad, F.; Knowlton, S.; Tasoglu, S. Paper-based Assays for Urine Analysis. Biomicrofluidics 2017, 11, 051501. (3) Wang, Z.; Zhang, J.; Liu, L.; Wu, X.; Kuang, H.; Xu, C.; Xu, L. A Colorimetric Paper-based Sensor for Toltrazuril and its Metabolites in Feed, Chicken, and Egg samples. Food Chem. 2019, 276, 707-713. (4) Wu, M.; Lai, Q.; Ju, Q.; Li, L.; Yu, H.-D.; Huang, W. Paper-based Fluorogenic Devices for in Vitro Diagnostics. Biosens. Bioelectron. 2018, 102, 256-266. (5) Liu, B.; Ni, H.; Zhang, D.; Wang, D.; Fu, D.; Chen, H.; Gu, Z.; Zhao, X. Ultrasensitive Detection of Protein with Wide Linear Dynamic Range Based on Core-Shell SERS Nanotags and Photonic Crystal Beads. ACS Sens. 2017, 2, 1035-1043. (6) Leng, Y.; Sun, K.; Chen, X.; Li, W. Suspension Arrays Based on Nanoparticle-Encoded Microspheres for High-Throughput Multiplexed Detection. Chem. Soc. Rev. 2015, 44, 5552-5595. (7) Andreou, C.; Kishore, S. A.; Kircher, M. F. Surface-Enhanced Raman Spectroscopy: A New Modality for Cancer Imaging. J. Nucl. Med. 2015, 56, 1295-1299. (8) Hakonen, A.; Andersson, P. O.; Schmidt, M. S.; Rindzevicius, T.; Kall, M. Explosive and Chemical Threat Detection by Surface-Enhanced Raman Scattering: A Review. Anal. Chim. Acta 2015, 893, 1-13. (9) Hao, J.; Han, M.-J.; Han, S.; Meng, X.; Su, T.-L.; Wang, Q. K. SERS Detection of Arsenic in Water: A Review. J. Environ. Sci-China 2015, 36, 152-162. (10) Fu, X.; Chen, L.; Choo, J. Optical Nanoprobes for Ultrasensitive Immunoassay. Anal. Chem.


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For TOC only


TiO2-Coated Silica Photonic Crystal Capillaries for Plasmon-Free

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