Single Plasmonic Particle with Exposed Sensing Hot Spot for

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Single Plasmonic Particle with Exposed Sensing Hot Spot for Exploring Gas Molecule Adsorption in Nano-localized Space Yangyang Wang, Xuemeng Li, Zhenning Su, Hao Wang, Hongqi Xia, Huanjun Chen, and Jianhua Zhou Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05653 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Analytical Chemistry

Single Plasmonic Particle with Exposed Sensing Hot Spot for Exploring Gas Molecule Adsorption in Nano-localized Space

Yangyang Wang,a Xuemeng Li,a* Zhenning Su,a Hao Wang,b Hongqi Xia,a Huanjun Chenb and Jianhua Zhou a*

a Key Laboratory of Sensing Technology and Biomedical Instruments of Guangdong Province, School of Biomedical Engineering, Sun Yat-sen University, Guangzhou 510275, China

b State Key Lab of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China

*Corresponding author: Tel.: +86 20 39387890; Fax: +86 20 39387890. Email: [email protected] (X. M. Li); [email protected] (J. H. Zhou).

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Abstract Single particle (SP) sensing technology provides a methodology to explore the biochemical process in a micro/nanosize area (super-high resolution) with high sensitivity. Plasmonic nanoparticle is promising to be utilized as a substrate for single particle sensing. To realize specific sensing, a modification layer on the surface of the plasmonic nanoparticle is usually in need. However, a challenge stands in the way: the traditional coating of modification layer can deplete the highly enhanced electric field (EF) around the plasmonic particle, and also perhaps hinder the analytes to move into the sensing hot spot with the most enhanced EF; thereby, the plasmonic particle can not perform a super-high sensitivity. To solve this problem, we demonstrated an innovative single plasmonic particle sensing system in this work. In a convenient and controllable way, a single gold nanorod (AuNR) was successfully modified by monolayer WS2. There is an energy interaction between the AuNR and WS2, and thus an exposed sensing hot spot with a non-depleted enhanced EF exists at the interface, which equips the as-prepared AuNR-WS2 SP with the ability to detect small changes of the local dielectric environment. Meanwhile, the monolayer WS2 also acted as a specific modification layer for detecting different analytes. We applied the AuNR-WS2 SP to explore the adsorption kinetics of different gas molecules including ammonia, ethanol and acetone for the first time. Through monitoring scattering spectra under a microscope in dark-field, AuNR-WS2 SP could successfully differentiate the three small molecules, and help to explore the adsorption kinetics of them. Our experimental results were consistent with theoretical simulation in SP’s EF distribution and its scattering spectra under different dielectric environments. Additionally, this proposed interaction-based modification strategy was also applied to other plasmonic nanoparticles such as Au@Ag nanocube and Au nanodisk, suggesting the universality of this innovative SP sensing system.

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Keywords: Single plasmonic nanoparticle, WS2 monolayer, gas sensing, modification layer,

adsorption

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kinetics


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Single particle (SP) sensing has attracted increasing attention because it can rule out the average effect from the ensemble measurements and provide valuable information

in

a

super-localized

space.1-3

This

space

advantage

enables

high-resolution bioimaging and ultra-high sensitive biosensing, thus paving the way to explore the principles behind biomolecule’s behavior and function.4-5 Among various SP sensing systems, plasmonic nanoparticle based on metal nanostructures is serving as a popular sensing platform because of its convenient fabrication, high sensitivity,

high

spatial

resolution,

(non-bleaching), and label-free etc.3,

excellent

6-16

signal-noise

ratio,

stability

SP scattering spectroscopy is the common

method to collect optical signals of SP sensing systems. Plenty of excellent works on plasmonic nanoparticles have been done to explore the biomolecule recognition process, such as enzyme catalysis17-22, antibody-antigen recognition23, and nucleic acid paring24, ion adsorption25, as well as to monitor the biochemical reaction in a cell26-27 etc. However, there is still a challenge impeding the progress of plasmonic SP sensing at least. Usually, a modification layer on the surface of a plasmonic SP can enable the nanoparticle to specifically capture target molecules. But the modification layer coated on the surface of plasmonic nanoparticles can occupy the space which contains an enhanced electric field (EF) deriving from the localized surface plasmon resonance (LSPR) of the plasmonic nanoparticle. This can lead to two problems. On one hand, the majority of the enhanced EF would be depleted by the over-thick modification layer; thereby, the refractive index (RI) sensitivity of the nanoparticle would be sacrificed.28 On the other hand, a coating modification layer could obstruct the diffusion of the target analytes, which would lower down the efficiency of the targets moving to the enhanced electric field region near the surface of the plasmonic nanoparticle.29-32 Such problems hinder the application of plasmonic nanoparticles into the detection of small molecules, especially small gas molecules.33-38 Therefore,

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an innovative SP sensing system, whose enhanced EF is not depleted by the modification layer and its sensing hot spot is exposed to the target analytes, is a desperate need. Herein, we proposed an innovative single plasmonic nanoparticle sensing system, which was fabricated through an “interaction-based partial modification” strategy. In this system, there should be an energy interaction between the plasmonic nanoparticle and the modification layer. Then the enhanced EF at the interface can provide an exposed sensing hot spot, which can be reached by small analytes and highly sensitive to small variation of the local dielectric environment. At the meanwhile, the modification layer is also supposed to help realize specific sensing. To demonstrate this novel SP sensing system, a single gold nanorod (AuNR) was partially modified by 2-dimensional (2D) semi-conductor WS2 monolayer through simply dropping the AuNR onto this 2D material of WS2. In this convenient way, the thickness control of the WS2 modification layer is no longer a problem, because mature techniques have already been existent to produce 2D transition metal disulfide ranging from monolayer to multilayer.39 What’s more, the highly enhanced EF is located at the interface between the AuNR and the monolayer WS2, which means the enhanced EF deriving from LSPR would not be sacrificed by the modification layer at all. Thus, with the assistance of this exposed sensing hot spot, the desirable high sensitivity of a single AuNR can be preserved (Scheme 1A).40-41 This new SP sensing system was highly sensitive to the change of dielectric environment, and was able to explore the adsorption kinetics of three different gases including ammonia, ethanol and acetone in nanoscale super-localized space for the first time. The scattering spectra of AuNR-WS2 SP were monitored in dark-field, and the shape variation of the spectra along with time could provide the information of the small molecule’s adsorption onto AuNR-WS2 SP (Scheme 1B). The strategy of interaction-based partial modification was also applied to other plasmonic NPs such as Au@Ag nanocube (Au@Ag NC) and Au nanodisk (AuND). The results suggest that fabricating a modification layer

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which can interact with the plasmonic nanoparticles can be an innovative way to develop highly sensitive sensing systems, which will broadly branch out the application of SP sensing technology.

Scheme 1. (A) Partially modify a single plasmonic nanoparticle with monolayer WS2. There is an energy interaction between the monolayer WS2 and the Au nanorod (AuNR), bringing about a super-localized enhanced electric field at the interface and thus an exposed sensing hot spot for small molecules. (B) The single AuNR-WS2 particle (AuNR-WS2 SP) can response to the adsorption of gas molecules, which makes it possible to explore small molecules’ adsorption behaviors by analyzing the kinetic curves.

Results and Discussion. Fabrication of AuNR-WS2 SP. AuNRs were synthesized through the seed mediated method. The original longitude LSPR wavelength of the as-grown AuNRs was about 720 nm. Then the as-grown AuNRs went through anisotropic oxidation and the wavelength can be tuned from ~720 nm to ~550 nm (Figure S1). Afterwards, the AuNR solution of 5 μL was dropped onto the surface of the single-crystalline monolayer WS2 on Si/SiO2 substrates (Shenzhen SixCarbon Company), and blown dry with nitrogen in 10 s. Finally, the AuNR was partially modified with monolayer WS2, and AuNR-WS2 SPs were obtained (Figure 1A). Since the monolayer WS2’s emission peak located at ~620 nm (Figure 1F), we synthesized AuNRs with LSPR wavelength at ~620 nm. The aspect ratio of such AuNRs was ~2 (Figure 1B). The hot 6 / 26


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electrons of AuNR can interact with the excitons of some transition-metal-based materials.42-43 According to previous reports40, when the LSPR wavelength of AuNR matches the photoluminescence emission peak wavelength of monolayer WS2, the AuNR and WS2 will form a coupling system, which could be highly sensitive to the environmental variations in the vicinity of the AuNR-WS2 SP. As reported in previous work about the coupling between semiconductors and plasmonic NPs, it can be deduced that the excitons of monolayer WS2 can interact with AuNR more strongly than multilayer and ordinary bulk WS2.44 Since the enhanced electric field at the interface between WS2 and AuNR mainly derives from their interaction, we chose monolayer WS2 as the modification layer in this work. To realize SP sensing, we employed the dark-field scattering spectroscopy in this work to characterize the AuNR-WS2 SPs. Figure 1C displays the triangular monolayer WS2 flake under a microscope in bright-field. Figure 1D displays the dark-field microscope image of a monolayer WS2 flake with dispersed AuNRs on it. In dark-field, different particles displayed as various luminous points. The scattering color of our as-prepared AuNR-WS2 SPs was observed as orange. The boundary of a monolayer WS2 flake could also be observed in dark-field, which helped to decide which AuNRs were partially modified by monolayer WS2 and which were not. The luminous points outside the triangular boundary represent the nanoparticles that located on the Si/SiO2 substrate, which were not modified by WS2. The scanning electron microscope (SEM) images of a monolayer WS2 flake with AuNR SPs on it indicates that a high degree of dispersion could be reached by our innovative partial modification method (Figure S2). The dark-field scattering spectrum of a single AuNR holds a peak width as narrow as ~50 nm.23, 45 This principle was employed in our judgement of a luminous point as a single AuNR or not. The scattering spectra of a AuNR and AuNR-WS2 SP are shown as Figure 1E and G, respectively. It can be obviously observed that the AuNR-WS2 SP system exhibits a scattering spectrum with a dip at the wavelength of the WS2’s emission peak, which can be an evidence that the targeted highly-sensitive

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coupling SP sensing system has been successfully fabricated by the partial modification. As shown in Figure 1G, two scattering peaks can be observed, which are defined as peak 1 and peak 2 in this work, respectively. We employed the ratio of peak 2’s scattering intensity and peak 1’s scattering intensity, which was marked as I2/I1, to characterize the scattering spectra of AuNR-WS2 SP.

Figure 1. Fabrication of AuNR-WS2 SPs. (A) The AuNR solution is dropped onto monolayer WS2 and dried out afterwards. The AuNRs were attached to the surface of monolayer WS2 mainly by Van der Waals force. (B) Transmission electron microscope (TEM) image of a AuNR SP, with a length of ~90 nm and a width of ~45 nm. (C) Bright-field microscope image of a monolayer WS2 flake. (D) Dark-field microscope image of a monolayer WS2 flake with dispersed AuNRs on it. (E) Dark-field scattering spectrum of a AuNR SP. (F) Photoluminescence spectrum of monolayer WS2 (λext=532 nm). (G) Dark-field scattering spectrum of a AuNR-WS2 SP. A dip can be observed in the AuNR-WS2 SP’s scattering spectra, whose position is at the wavelength of monolayer WS2’s emission peak, and this can be an evidence of the energy interaction between 8 / 26


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AuNR and monolayer WS2. The scattering intensity of the two peaks are defined as I1 and I2. The value of I2/I1 is employed to characterize each spectrum. The scale bars in (B) and (C) are 50 nm and 10 μm, respectively.

Performance of the AuNR-WS2 SP sensing system. To examine the stability of this AuNR-WS2 SP sensing system, we monitored the scattering spectra of one AuNR-WS2 SP in the laboratory’s atmosphere for nearly one hour (Figure 2A). The value of I2/I1 was extracted from each spectrum. As shown in Figure 2B and Figure S3, the performance of one AuNR-WS2 SP and AuNR SP both exhibited no obvious change in atmosphere during one hour. Theoretical simulations were conducted by the FDTD method to explore the electric field distribution of a AuNR-WS2 SP. As reported, there is an energy interaction between plasmonic nanoparticles and some monolayer semiconductors.40-41 The monolayer WS2 in our AuNR-WS2 SP system served not only as a specific modification layer, but also interacts with the AuNR. As shown in Figure 2C, the highly enhanced localized EF exists at the interface between the AuNR and monolayer WS2. Because the ~1 nm surfactant layer of CTAB around the AuNR adsorbed in the synthesis process can be remained even after the purification process, there is a ~1 nm gap between the AuNR and the monolayer WS2, which allows small molecules to move into the exposed sensing hot spot which locates at the interface with highly enhanced electric field (Figure 2D).40, 46 Therefore, the simulation proves that our innovative SP sensing system with an interaction-based partial modification layer holds the ability to preserve the enhanced electric field and this sensing hot spot can be exposed to target analytes. This makes our AuNR-WS2 SP hold the potential to sensitively detect small molecules’ adsorption. Because WS2 is sensitive to some polar molecules, it is worth to explore the response of the AuNR-WS2 SP to ethanol gas molecules.41 We designed and fabricated a microfluidic chip which can expose SPs to one specific gas environment (Figure 2E). There is a reservoir to hold volatile liquids, which allows the gas molecules to be evaporated to

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the whole space of the sensing chamber (Figure S4). It is apparently observed that after incubated in saturated ethanol gas, the scattering spectra of the AuNR-WS2 SP exhibited an enormous change that I1 became weaker and I2 became stronger, which makes the value of I2/I1 became larger compared to the original scattering spectrum in atmosphere (Figure 2F). This phenomenon proves that the AuNR-WS2 SP holds potential to be used in small molecule sensing.

Figure 2. (A) Scattering spectra of a AuNR-WS2 SP monitored in the laboratory’s atmosphere for ~ 1 h. (B) The values of I2/I1 extracted from the spectra in (A) were almost unchanged during the monitoring process. (C) Electric field (EF) distribution of a AuNR-WS2 SP theoretically simulated by the FDTD method, which indicates the 10 / 26


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enhanced EF is super-localized in a small space at the interface between the AuNR and monolayer WS2. (D) The exposed sensing hot spot at the interface with a highly enhanced EF can be reached by small molecules. (E) Scheme of a microfluidic chip which can expose the single particle to a saturated gas environment. (F) The scattering spectra of a AuNR-WS2 SP before (black) and after (red) exposing to a kind of polar molecules, ethanol.

Exploring the adsorption kinetics of different gas molecules. With the help of the as-fabricated microfluidic chip, we could explore the response of AuNR-WS2 SPs to different small gas molecules. The scattering spectra of AuNR-WS2 SPs in ammonia, ethanol and acetone were monitored respectively. The Raman scattering spectra of monolayer WS2 in Figure S5 shows that the characteristic peaks of monolayer WS2 at 178.1 cm-1 (

), 356.8 cm-1 (

) and 419.3 cm-1 (

) didn’t change

before and after exposed to the three saturated gases47, which indicates that no obvious structural damage of the partial modification layer happened during the gas incubation process. Figure 3A-C shows the scattering spectra before (black) and after (red) the gas incubation. Other spectra monitored at different time points during the incubation are shown in Figure S6. In all the three monitoring experiments, it is observed that I1 of the AuNR-WS2 SP’s scattering spectra decreased after the gas incubation and I2 increased after the gas incubation. What’s more, we noticed that this SP sensing enabled us to explore the adsorption behavior of small gas molecules in a nanoscale super-localized space. To explore the adsorption behavior of these three different gas molecules, the value of I2/I1 was extracted from each spectrum and we recorded the change of I2/I1 along with time (Figure 3a-c). The adsorption behavior of molecules is usually divided into the first-order (mainly determined by the physical diffusion) and the second-order (influenced by chemical affinity, such as electron sharing and electron

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transfer) adsorption kinetics. We conducted first-order fitting by Lagergren function, and second-order fitting by Mckay and Elovich functions. The functional equations are displayed in the Supporting Information. In this work, the variation of the SP scattering spectra is derived from the RI change in the vicinity of the SP, especially at the interface with a highly enhanced electric field. In our experiment, the adsorption of molecules onto the SP contributes most to the localized RI change. Therefore, it is reasonable to represent the adsorption quantity of molecules by the I2/I1 value extracted from the scattering spectra of the AuNR-WS2 SP. It was found that in our AuNR-WS2 SP sensing system, the adsorption kinetics of ammonia and ethanol gas molecules fit Mckay functions (second-order) best. As for the acetone gas molecules, their adsorption kinetics matched Lagergren fitting (first-order) most. Especially, the adsorption of acetone gas molecules onto AuNR-WS2 SP cannot match Elovich fitting (second-order) at all (Figure S7). It would be a reasonable deduction that the ammonia and ethanol adsorptions onto AuNR-WS2 SP were influenced by the WS2’s chemical affinity to polar molecules. The monolayer WS2 served as the modification layer in our SP sensing system, which could enable the SP to distinguish small molecules with different polarity. It is commonly known that the polarity order goes as follows: ammonia > ethanol > acetone. In the meanwhile, monolayer WS2 displays sensitivity to polar molecules. When gas molecules with different polarity adsorbed onto the monolayer WS2, the coupling between WS2 and AuNR would be varied with the change of the energy level of WS2, resulting in the spectral variation of SP’s scattering. The difference of ammonia’s and ethanol’s polarity can probably explain why the curves of their second-order adsorption kinetics look obviously different and why the acetone adsorption fits the first-order adsorption kinetics best. Additionally, the enhanced electric field of the SP was not sacrificed by the modification layer of WS2, which made our AuNR-WS2 SP sensitive enough to detect small gas molecules’ adsorption process and distinguish them in nanoscale space. In the AuNR SP sensing system, we

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extracted the peak position from the spectra and calculated the peak shift at each time point. We also conducted all the as-mentioned function fittings to the “peak shift-time” curve (Figure S8). The results showed that the adsorption kinetics of ammonia, ethanol and acetone gas molecules all matched Lagergren fitting (first-order) best, with no obvious variation in the fitting curves. We also conducted theoretical simulation of a AuNR and AuNR-WS2 SP’s scattering spectrum in dielectric environments with different RI (n=1.0 and n=1.1) by the FDTD method (Figure S9). It can be observed that the shape variation pattern of the spectra was highly consistent with our experimental results. The simulated spectra by FDTD only depended on the parameters that we set; thereby, the differences between the simulated and experimental results could be derived from plenty of reasons. For one thing, the RI change was the only factor that caused the spectral variation in simulation, while the gas molecules’ polarity also affected the SPs’ spectra in the experiments. For another, the fluctuation of the humidity, the atmosphere pressure and the temperature etc. could also lead to the differences of the sensing performance between the perfectly simulated AuNR-WS2 and that in the real sensing environment.

Figure 3. (A-C) Scattering spectra of a AuNR-WS2 SP before (black) and after (red and blue lines) exposing to (A) ammonia, (B) ethanol and (C) acetone gas, respectively. (a-c)

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The time-dependent value of I2/I1 was extracted from each spectrum. The relationships of I2/I1 and time were fitted to explore the adsorption kinetics of these three different gas molecules. The adsorption of (a) ammonia and (b) ethanol gas molecules onto AuNR-WS2 SP matched Mckay function (a second-order kinetic fitting) most, which means their adsorptions were influenced by chemical affinity. The adsorption of (c) acetone gas molecules onto AuNR-WS2 SP matched Lagergren function (a first-order kinetic fitting) most, which means their adsorptions were mainly determined by the physical diffusion.

To demonstrate the broad applicability of the interaction-based partial modification, we branched out to other plasmonic nanoparticles, including Au@Ag NC (Figure S10) and AuND (Figure S11). These two plasmonic SPs were successfully modified by monolayer WS2, showing a dip at the wavelength of ~620 nm in their scattering spectra. The as-fabricated Au@Ag NC-WS2 and AuND-WS2 SPs were monitored in the atmosphere for nearly an hour. The shape of the spectra showed no obvious variation during the monitoring period, which proved the stability of these SPs. We also demonstrated their scattering spectra’s response to saturated ethanol gas, which indicates that these partially modified plasmonic SPs also have the sensing capability of exploring adsorption kinetics of small gas molecules. In conclusion, a novel system of single particle plasmonic sensing has been successfully demonstrated by a single AuNR partially modified with monolayer WS2. The monolayer WS2 serves not only as a specific layer to distinguish molecule’s polarity, but also as a part which can interact with the AuNR. This interaction brings out a highly enhanced electric field at the interface of AuNR and WS2, which equipped the AuNR-WS2 with an exposed sensing hot spot. This novel sensing system is sensitive to three different small gas molecules and helps to explore the different adsorption behaviors of these molecules in nanoscale super-localized space for the first time. Compared to other modification methods for plasmonic sensing, this interaction-based partial modification strategy is convenient and controllable, and 14 / 26


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additionally plasmonic nanoparticles can be modified in a more uniform way. This strategy can also apply to other plasmonic nanoparticles, such as Au@Ag NC and Au ND. Single particle sensing is known for its ultra-high sensitivity, but still difficult to distinguish different small molecules. In future, this interaction-based partial modification strategy can perhaps introduce other 2D materials and super materials (such as semi-conductors48-53 and metal-organic frameworks54) which might interact with plasmonic nanoparticles and simultaneously serve as a specific identifying role, to modify plasmonic nanostructures. The energy interaction between the plasmonic nanoparticles and the modification layer can enhance the local electric field at the interface. This exposed and non-depleted sensing hot spot will improve the accuracy of distinguishing similar molecules and broaden the application of plasmonic sensing in the research of biochemical reaction process, interaction between biomolecules, and

high

space-resolution

detection

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or

sensing

etc..


Materials and Methods Materials. Gold (III) chloride trihydrate (HAuCl4•3H2O, >99.0%), sodium borohydride (NaBH4, 99%), were purchased from Aladdin Chemical Co., Ltd. (Shanghai, China); cetyltrimethylammonium bromide (CTAB, 99%), trisodium citrate (C6H5O7Na3•2H2O, 99.00%), HCl (36%) were purchased from Damao Chemical Co., Ltd. (Tianjin, China). Deionized water was prepared by a Milli-Q Advantage A10 water system (Millipore, Billerica, MA, USA) with a resistivity of 18.2 MΩ•cm. Polydimethylsiloxane (PDMS, including component A and B) was bought from Dow Corning, USA. Spectroscopy. The extinction spectra of the AuNR solution were obtained using a UV-visible spectrophotometer (Inesa L3S). The photoluminescence and Raman spectra of the monolayer WS2 were collected using a Renishaw inVia Reflex system with an excitation laser of 532 nm with a diameter of ∼1 μm through a 50× objective (numerical aperture 0.80). The single particle scattering spectra were recorded on a home-built dark-field microscope. The optical microscope (Olympus BX51) was integrated with a quartz-tungsten-halogen lamp (100 W), a monochromator (Acton SpectraPro 2360), and a charge-coupled device camera (Princeton Instruments Pixis 400BR_eXcelon). The camera was cooled down to -70 °C during the measurements thermoelectrically. A dark-field 50× objective (numerical aperture 0.80) was employed for both illuminating the heterostructures at an incidence angle of ∼55° with the white light and collecting the scattered light. Characterizations. SEM images were acquired through an FEI Quanta 450 microscope. Morphology of a single AuNR was characterized by TEM (JEM-1400) with an operation voltage of 120 kV. Theoretical simulations. The finite-difference time-domain (FDTD) method was utilized to calculate the electric field distribution and scattering spectra of AuNR-WS2 SPs. The AuNR was modeled as a cylinder capped with a hemisphere at each end and

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located on a monolayer WS2 on top of SiO2 substrate. The diameter of the AuNR was set as 45 nm and the length was set as 90 nm. The gap between the nanorod and the monolayer WS2 was set as 1 nm. The thickness of the monolayer WS2 was kept as 1 nm. The dielectric functions of the gold and monolayer WS2 were previous reports.55,56 A dielectric constant of 2.25 was used for the SiO2 substrate. A mesh size of 0.5 nm was set around the nanorods.

Supporting Information Extinction spectra of AuNR solutions monitored during the anisotropic oxidation process. SEM images of monolayer WS2 with dispersed AuNRs. Stability of AuNR SP in atmosphere. The structure of the microfluidic chip. Raman scattering spectra of monolayer WS2 before and after the gas incubation. All scattering spectra of AuNR SP and AuNR-WS2 SP monitored at different time points during the gas incubation. Fitting functions of the adsorption kinetics. Simulated spectra by the FDTD method. Response of Au@Ag NC-WS2 SP and AuND-WS2 SP to the ethanol gas.

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Acknowledgments This work was supported in part by the National Natural Science Foundation of China (No. 21775168, 11474364 and 51290271), National Key R&D Program of China (2017YFE0102400), and Department of Science and Technology of Guangdong Province (2017A020211004). The work was also supported in part by the Australia-China Joint Institute for Health Technology and Innovation.

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Single Plasmonic Particle with Exposed Sensing Hot Spot for

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