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Abstract
Exploring multifunctional surface-enhanced Raman scattering (SERS) substrates with high sensitivity, broadband response property and reliable practicability should be required for ultrasensitive molecular detection in complex environments, which is heavily dependent on the photo-induced charge transfer (PICT) efficiency realized on the desirable nano-architectures. Herein, we introduce ultra-clean ternary Au/Ag/AgCl nanoclusters (NCs) with broadband resonance crossing the visible light to near-infrared region created by one step laser irradiation of mixed metal ion solution. Interestingly, the surface defects and interaction among these unique cluster-like ternary nanostructures would be further enhanced by thermal annealing treatment at 300°C, providing higher broadband SERS activities than the reference ternary nanoparticles under 457, 532, 633, 785, and 1064 nm wavelengths excitation. More importantly, the further promoted SERS activities of the resultant Au/Ag/AgCl NCs with achievable ∼5-fold enhancement than the initial one can be conventionally realized by simplistically declining the temperature from normal 20°C to cryogenic condition at about -196°C, due to the lower temperature-suppressed non-radiative recombination of lattice thermal phonons and photogenerated electrons. The cryogenic temperature-boosted SERS of the resultant Au/Ag/AgCl NCs enables the limit of detection (LOD) of folic acid (FA) biomolecules to be achieved as low as 10−12 M, which is obviously better than that of 10−9 M at room temperature condition. Overall, the smart Au/Ag/AgCl NCs-based broadband SERS sensor provides a new avenue for ultrasensitive biomolecular monitoring at cryogenic condition.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) spectroscopy owing to inherent ultra-sensitivity, high selectivity, non-destructive detection and other merits, has developed into a powerful trace-molecular analysis technology, which has been extensively implemented in the wide fields of biomedicine, food safety, environmental pollution, etc. [1–5]. To date, the proposed electromagnetic enhancement (EM) and chemical enhancement (CM) are considered to be two widely accepted mechanisms for SERS enhancement [4–6]. Generally, the EM mechanism is mainly related to the enhanced surface electromagnetic intensity produced by the localized surface plasmon resonance (LSPR) on the rough surface of plasmonic metallic nanostructures. So, the resultant high-performance LSPR promotes the efficient transmission/separation of energetic charge carriers between metal SERS substrates and target molecules, enabling the Raman signals to be significantly amplified up to 106 or even higher. Meanwhile, the CM is established by the photo-induced charge transfer between the molecules and semiconductor substrates under the resonant laser excitation, giving rise to the enhanced molecular polarization. Consequently, abundant novel metal-based nanocomposites substrates like MXene/Au nanorods [7], three-dimensional (3D) flower-like Ag@ZnSe composites [8], Au nanocrystals on suspended single-walled carbon nanotubes [9], etc., have been successfully designed to improve the SERS activity maximally via proposing the combination of EM and CM, propelling ultra-sensitive detection of analytes even at the single-molecule level [10,11]. Importantly, it should be noted that both EM and CM enhancement of composite SERS nano-substrates should be heavily dependent on the photo-induced charge transfer (PICT) efficiency within the substrate-molecule system [12–14]. The higher PICT efficiency not only facilitates more electrons to participate the construction of local electromagnetic field but also optimizes the charge transition probability between substrates and molecules via the generation of significant excited electron density, resulting in greatly increased SERS sensitivity. Therefore, improving the PICT efficiency is the key factor for boosting SERS activity, which is still at the early state of exploration up to now.
With the increase of higher demand for ultrasensitive SERS, several intriguing strategies have been explored to rationally improve the PICT efficiency, including defect engineering, ultraviolet irradiation and electric field adjusting [15–17]. Compared with those traditional proposed methods by increasing the charge transfer pathways and/or enhancing vibronic coupling effect within the substrates-molecule system, the suppressing non-radiative recombination of lattice thermal phonons and photoexcited electrons will be also essentially beneficial to elevate the PICT efficiency. Recently, many interesting reports have verified that the adopted lower temperature can effectively weaken the lattice thermal vibration and suppress phonon-assisted relaxation, thereby reducing the phonon-assisted non-radiative recombination [18–23]. Especially, Lounis et al. [19] provided an intense exciton emission in the low-temperature photoluminescence (PL) of single formamidinium lead bromide (FAPbBr3) nanocrystals under magnetic fields because of an extremely reduced phonon-assisted relaxation. Meanwhile, Mahamuni et al. [21] discovered that PL emission intensity of cesium lead chloride (CsPbBr3) nanocrystals achieved 90-fold improvement at low temperature of 20 K attributable to reduced carrier traps, leading to an excitonic superradiance effect. Inspired by the low temperature-enhanced PICT efficiency, Lin et al. [22] successfully realized low temperature-boosted high efficiency PICT by effectively suppressing non-radiative recombination, thus enabling remarkably enhanced SERS activity for porous d-ZnO nanosheets. Similarly, Ma et al. [23] developed novel Au/Ag nano-urchins as an outstanding SERS substrate and promoted their near-infrared SERS (NIR-SERS) activity by boosting the PICT efficiency at cryogenic temperature. As a result, using low temperature would provide a promising strategy to facilitate the PICT efficiency essentially, and then effectively improve the SERS sensitivity. At the present time, previous researches of cryogenic temperature-boosted SERS have solely focused on the bare semiconductor or pure plasmonic nano-structures, and there is very rare report on the metal/semiconductor nanocomposites that can facilitate higher PICT efficiency.
On the other hand, the emerging broadband responsive SERS with more practicability and versatility under multiple excitation wavelengths has attracted growing attention in compassion with single wavelength excitation, which can robustly provide multiplexed detection of multiple analytes in real-world scenarios [24–28]. In this regard, it is also highly desirable to rationally construct appropriate SERS substrates for broadband SERS response under different laser excitations. Many researches have proved that exploring novel SERS nanostructures with broaden optical absorption is a primary prerequisite for obtaining good broadband SERS response [25–28]. For instance, Zhang et al. [26] skillfully designed 3D hierarchical plasmonic structures with excellent optical absorbance in a wide wavelength range (350∼1000 nm), thus realizing high and stable Raman performance at different excitation wavelengths. Dey et al. [27] successfully fabricated Au nano-assemblies with an intense LSPR across 650∼1100 nm as a novel SERS substrate candidate for boosting broadband NIR plasmon coupling. Mao et al. [28] proposed a broadband SERS substrate composed of 3D Ag hybrid nanoaggregates with a broadband plasmonic resonance in the visible spectral range (400∼800 nm). Considering that optical properties of nanostructures are drastically influenced by their morphologies [29–31], nanomaterials with desirable optical properties can be easily achieving by regulating the morphology of nanomaterials. Inspired by above discussions, various one-dimensional (1D) nanoclusters have been exploited to realize broadband absorption and conversion, since the cluster-like distributed hot spots derived from longitudinal plasmon coupling can broaden the optical absorption bands dramatically [32–34]. Noticeably, the unique cluster-like nanostructure can also promote charge transfer between the SERS substrates and target molecules [35,36], thereby benefiting to improve the PICT efficiency. Despite these great achievements obtained in most of previous efforts, however, the broadband responsive SERS performances are still far from satisfactory toward the growing demand of ultrasensitive molecular detections, because it is very difficult to simultaneously enhance multiple SERS activities on the same broadband nano-substrates.
In this work, we proposed a multi-resonant nano-substrate with simultaneously remarkably boosted SERS activities under broadband visible-NIR laser excitations based on the construction of ternary Au/Ag/AgCl NCs. The achievable ∼5-fold enhancement of SERS signals can be conventionally realized by simplistically declining the temperature from normal 20°C to cryogenic condition at -196°C. The mentioned Au/Ag/AgCl NCs with ultra-clean surfaces were successfully synthesized by a green and simple one-step strategy via 532 nm pulsed laser irradiation of AgNO3 and HAuCl4 metallic mixed solution. Because of the unique cluster-like 1D nanostructures, the resultant Au/Ag/AgCl NCs with a high absorbance capacity across the visible to near-infrared regions (400∼1100 nm) provide remarkable broadband SERS features under 457, 532, 633, 785 and 1064 nm excitation wavelengths in comparison with the traditional ternary nanoparticles. Interestingly, the surface defects and interaction among the ternary components in the as-prepared Au/Ag/AgCl NCs can be effectively enhanced by thermal annealing treatment at 300°C condition, giving rise to the over 3-fold higher SERS activity than unannealed sample. More importantly, the cryogenic temperature-boosted SERS of folic acid (FA) biomolecules adsorbed on the resultant Au/Ag/AgCl NCs facilitates the limit of detection (LOD) to be achieved as low as 10−12 M, which is much better than that of 10−9 M detected at normal room temperature. The broadband resonant Au/Ag/AgCl NCs with cryogenic temperature-triggered ultrahigh multiple SERS activities under visible-NIR excitations are rationally expected to provide an attractive pathway for robust ultrasensitive detections of biomolecules in complex conditions.
2. Experimental details
2.1 Materials
Silver nitrate (AgNO3, 99.9999%), chloroauric acid (HAuCl4, 99.9%) and sodium chloride (NaCl, 99.99%) were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Rhodamine 6 G (R6G, 95%) and folic acid (FA, 97%) were obtained from Macklin Biochemical Industry Co., Ltd (Shanghai, China). Ethanol was bought from Tianjin Fuyu Chemical Co., Ltd. All reagents were used without further purification. Ultrapure water used in the experiments was prepared using a Millipore purification system (18.2 MΩ cm).
2.2 Preparation of ternary Au/Ag/AgCl NCs
In the traditional chemical synthesis of hybrid nanocomposites, various complex organic regents such as cetyltrimethylammonium (CTAB) [37], 2-mercaptopyridine (2-MPy) [38], poly(n-vinyl-2-pyrrolidone) (PVP) [39], etc. are introduced as stabilizers/capping/structure directing additives to adjust the growth of nanostructures, leading to the presence of an organic layer contamination on the surface of the hybrid nanomaterials. By comparison, without any organic additives, the ternary Au/Ag/AgCl NCs with ultra-clean surface were prepared via a simple and effective one-step laser irradiation synthesis route in the liquid surrounding, which is similar to our previous work [40]. Briefly, the reaction solution including 10 mM AgNO3, 5 mM HAuCl4 and 10 mL distilled water was irradiated by a Q-switched Nd-YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra Physics) beam. The laser beam operated at a wavelength of 532 nm with pulse duration of ∼6 ns, the energy of ∼200 mJ and 10 Hz repetition. Based on the previous discussion, the ternary Au/Ag/AgCl nanoparticles (NPs) would be preferentially fabricated attributed to the overgrowth of Au/Ag nanocrystals derived from the photoexcited electrons on semiconductor AgCl and the repetitive dissolution-recrystallization of AgCl precursors during multiple laser pulses. Then, the as-prepared NPs would be further interconnected with each other by subsequent laser sintering. After laser irradiation for 30 minutes, the ternary Au/Ag/AgCl NCs were successfully fabricated by carefully rinsing the reaction suspension with de-ionized water several times by centrifuging at 2000rpm for 10 minutes in an ultracentrifuge.
2.3 Preparation of the reference SERS substrates
For comparison, AgCl NPs, binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs were successfully fabricated and used as reference SERS substrates in the identical SERS test condition. In a typical fabrication procedure, AgCl NPs were facilely synthesized via using 10 mM AgNO3 and 10 mM NaCl as the precursor under the dark condition for 30 min. The binary Ag/AgCl NPs were obtained via using 10 mM AgNO3 and 10 mM NaCl as the precursor at the same laser irradiation condition under 30 min irradiation. Compared with the fabrication of ternary NCs, we successfully synthesized the ternary Au/Ag/AgCl NPs by decreasing the laser irradiation time and energy [40]. Similarly, the ternary Au/Ag/AgCl NPs were successfully fabricated via 532 nm pulsed laser with pulse duration of ∼6 ns, the energy of ∼150 mJ and 10 Hz repetition in the mixed solution including 10 mM AgNO3, 5 mM HAuCl4 and 10 mL distilled water for 20 min. Finally, the obtained AgCl NPs, binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs were washed with water to remove any unreacted starting materials via centrifugation at 5000 rpm for 10 min.
2.4 Materials characterization
The micro-morphologies and chemical composition investigations of as-prepared nano-products were performed by focused ion beam electron microscope (FIB, Helios G4 UC) equipped with energy-dispersive-x-ray spectroscopy (EDS). The Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM) images were recorded by JEOL JEM-2100F, with an accelerating voltage of 200 kV. Additionally, the surface compositions and element valences were recorded by X-ray photoelectron spectra (XPS) by a PHI Quantera SXM. The crystallographic tests were analyzed by X-ray diffraction (XRD) patterns (Rigaku, Smart Lab 9 kW) using Cu Kα radiation (λ=0.15406 nm) in the range of 10∼80°. The absorption spectra of the nanoproducts were taken from UV-Vis-IR spectrometer (UV-1800, Shimadzu). The element compositions among the nanoproducts were further determined by PerkinElmer NexION 300X inductively coupled plasma-mass spectrometry (ICP-MS). The PL spectra were collected using Renishaw high power diode Laser (Laser power: 1 mW; Lens: 50× objective; Acquisition time: 1 s) under the excitation wavelength of 633 nm. The cooling system utilized in temperature-dependent PL spectra measurements was realized by the additional Linkam Scientific Instruments Heating Cell kit with controllable temperature ranging from 20°C down to -196°C.
2.5 SERS measurements
All SERS signals were collected by a confocal microprobe Raman spectrometer (Renishaw Raman spectroscopy) to compare SERS performances of different nanostructures. The additional Linkam Scientific Instruments Heating Cell kit would control the temperature ranging from 20°C down to -196°C. SERS spectra were collected at different temperature using the excitation wavelength 457, 532, 633, 785 or 1064 nm laser with an output power of 1 mW as well as the acquisition time of 1 s. The lasers were focused onto a spot with approximately 1 µm2 using an objective microscopy of 50×. All the SERS spectra were recorded in a spectral range of 600 to 1800cm−1. According to previous reports [41,42], silicon plates were thoroughly cleaned to obtain a strong hydrophobicity surface and used as the substrates for SERS detection in the following. Firstly, the silicon wafers were cleaned ultrasonically for 10 min in acetone, then rinsed with ultrapure water for three times. Subsequently, the cleaned wafers were immersed in a mixture solution of H2SO4 (98%) and H2O2 (30%) with a volume ratio of 3:1 for 30 min, followed by rinsing with ultrapure water for several times. Finally, the silicon wafer with plentiful Si-H bonds formed on its surface was obtained by immersing it in 5% HF aqueous solution for 30 min and drying it under a stream of nitrogen. As a result, the surface of the treated silicon wafers was H-terminated and exhibited strong hydrophobicity. Then, the nanomaterials-based substrates were prepared by dropping 0.1 M/10 µL as-prepared nanoproducts on pretreated silicon plates and then dried naturally at room temperature for 12 h. In a typical sample droplet evaporation process, a smaller and thicker concentrated liquid drop would be formed due to a smaller contact between the sample droplets and pretreated hydrophobicity silicon wafers, thereby suppressing the coffee-ring effect and improving the uniformity of SERS substrates. A stock solution of 1 mM R6G/FA solution was prepared in ethanol and diluted further for the low concentrations. Next, the SERS substrates were immersed in various concentration of R6G/FA ethanol solution (1 mL) for 6 h and then washed by ethanol to remove unbound molecular and dried at nitrogen atmosphere before the SERS measurements. In order to ensure the test reproducibility, the SERS spectrums were recorded at different ten positions on each sample and the spacing between two points is about 2∼3 µm. Additionally, low magnification optical images (Fig. S1) were used to assist us to obtain a more uniform area from target molecules-nanoproducts SERS substrates in a typical SERS measurement.
3. Results and discussion
Initially, the transmission electron microscopy (TEM) measurements were carried out to discern the micro-morphologies of the as-obtained Au/Ag/AgCl NCs, as illustrated in Fig. 1. Obviously, the as-synthesized nanoparticles are connected with each other, forming a cluster-like shape, as described in Fig. 1(a). The inset in Fig. 1(a) shows an enlarged view of the blue rectangle marked in Fig. 1(a). Different from traditional separated nanospheres, the nanoparticles in these distinctive nanoclusters have inserted to each other, providing more convenient channels for the separation and migration of photoexcited carriers [35,43]. Figure 1(b) is the selected area electron diffraction (SAED) pattern of as-prepared nanoproducts, suggesting its inherent polycrystalline nature due to the existence of multiple diffraction rings. High-resolution TEM (HRTEM) image as shown in Fig. 1(c) reveals that lattice fringes of 0.28 and 0.24 nm are separately ascribed to AgCl (200) and bimetallic Au/Ag (111), revealing the formation of the ternary Au/Ag/AgCl nanostructures. As displayed in Fig. 1(d), the EDS line scanning profile and elemental distributions of an individual nanoparticle further intuitively depict that Au, Ag and Cl with relative atomic ratio of 8.1:49.8:42.1 are homogeneously distributed on the typical nanoparticle, confirming the obtained nanoproducts indeed composed of ternary Au/Ag/AgCl composites.
Fig. 1. The microstructure of ternary Au/Ag/AgCl NCs characterized by transmission electron microscopy: (a) TEM and high-magnification TEM (inset) images; (b) SAED pattern; (c) HRTEM image; (d) EDS line scanning and elemental distribution profiles.
To further corroborate Au/Ag alloy structures of the ternary Au/Ag/AgCl NCs, powder XRD were performed on the target, as shown in Fig. 2. All the diffraction peaks of the obtained ternary Au/Ag/AgCl NCs could be well matched with the standard PDF card in Fig. 2(a). More importantly, no other additional diffraction peaks could be observed in Fig. 2(a), indicating that the as-synthesized ternary nanoclusters have a high level of purity. Wherein, the XRD pattern of the ternary Au/Ag/AgCl NCs shows the coexistence cubic phases of AgCl (PDF#31-1238), Ag (PDF#04-0783) and Au (PDF#04-0784). Because the Au and Ag crystals possess very similar face-centered cubic crystal structures, their respective XRD patterns are hardly distinguished in the obtained ternary Au/Ag/AgCl nanoproducts. Subsequently, the magnified XRD patterns of (111) and (200) crystal planes located at 38.19° and 44.38° of ternary Au/Ag/AgCl NCs are illustrated in Fig. 2(b). Apparently, the two diffraction peaks located between the standard peak positions of pure Au and Ag phase, respectively, presenting a powerful evident for the formation of Au/Ag alloy structure in this work.
Fig. 2. (a) XRD pattern of the obtained ternary Au/Ag/AgCl NCs. (b) Magnified images of the characteristic diffraction peaks ascribed to (111) and (200) crystal planes.
Then, the scanning electron microscopy (SEM) images were used to investigate the microstructures of as-obtained nanoproducts in this work, as described in Fig. 3. Firstly, the low-magnification and enlarged SEM images displayed in Fig. 3(a)-3(b) are further demonstrated to manifest the as-prepared products as cluster-like nanostructures with connected nanoparticles. Based on the particle size distribution histogram (inset in Fig. 3(a)), the constituent nanoparticles in the ternary Au/Ag/AgCl NCs have a narrow size distribution with an average particle diameter of 428 ± 25 nm. The magnified SEM image is shown in Fig. 3(b), wherein representative connected cluster-like ternary Au/Ag/AgCl NCs are displayed clearly. The EDS analysis of nanoproducts as presented in Fig. 3(b) clearly verifies the definite compositions of as-obtained samples. The corresponding data shows that the EDS peaks of Au, Ag and Cl elements could be discerned distinctly, and the relative atomic ratio of Au:Ag:Cl is evaluated to be about 8.4:49.9:41.7, which is coincident with the calculated result in Fig. 1(d). Next, the element compositions among the ternary Au/Ag/AgCl NCs are further accurately determined by ICP-MS measurements, and the obtained atomic ratio of Au:Ag:Cl is about 8.3:50.2:41.5, confirming the validity of EDS results. Noticeably, the atomic ratio of Ag to Cl is appreciably larger than that of theoretical value of AgCl with stoichiometric atomic ratio of Ag to Cl ∼ 1:1, and the excess Ag should be originated from bimetallic component, implying that the nanoproducts should be composed of Au/Ag alloys and AgCl. In these cases, the overall atomic ratio of Au:Ag in Au/Ag alloy is about 1:1 according to EDS result (Au:Ag = 8.4:8.2). Clearly, the energy-dispersive X-ray (EDX) elemental mappings as represented in Fig. 3(c) reveal that Au, Ag, Cl maps are uniformly superimposed on the whole nanoclusters, further indicating the successful formation of ternary Au/Ag/AgCl NCs. Meanwhile, the typical SEM and EDS results of these three reference samples including AgCl NPs, binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs are displayed in Fig. 3(d)-3(f), and the EDS analysis of nanoproducts (insets) clearly corroborates the definite compositions of as-obtained samples.
Fig. 3. The microstructure of ternary Au/Ag/AgCl NCs characterized by scanning electron microscope: (a) low-magnification SEM image and size distribution histogram (inset); (b) high-magnification SEM image and corresponding EDS results (inset); (c) energy-dispersive X-ray (EDX) elemental mapping. (d)-(f) SEM images and corresponding EDS results (inset) of AgCl NPs, binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs, respectively.
Generally, the composition and morphology of the as-prepared samples can be also reflected by the solution color in a very intuitive way. As shown in Fig. 4(a), the pure AgCl NPs show the characteristic milk white color. After introducing metal species into AgCl samples, an obvious change in solution color could be observed and the two solutions exhibit wine red and purplish red, suggesting the formation of binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs, respectively. While for ternary Au/Ag/AgCl NCs, the dispersion color shifts to dark grey when the nanostructures changed from NPs to NCs. The inset in Fig. 4(a) demonstrates the powder of ternary Au/Ag/AgCl NCs after 300°C annealing exhibits bright red-brown color. As a result, we can modulate the growth of AgCl-based nanomaterials according to their colors in solution. Additionally, the typical SEM images of ternary Au/Ag/AgCl NCs after 300°C annealing evaluate that the unique cluster-like structure could be retained intactly after appropriate heat treatment process as shown in Fig. 4(b). Compared with the unannealed ternary nanoproducts (Fig. S2), the inset in Fig. 4(b) displays that a rough surface with abundant protrusions can be observed after 300°C annealing temperature, suggesting the recrystallization of the ternary Au/Ag/AgCl NCs under the thermal treatment. Owing to surface oxygen doping ascribing from the heat treatment in the non-vacuum environment, the corresponding HRTEM image in Fig. 4(c) reveals that abundant surface defects are formed in the resultant ternary Au/Ag/AgCl NCs annealed at 300°C in comparison to that of unannealed nanoproduct (Fig. 1(c)). However, repeated experiments have shown that the oxygen content can’t be detected from the EDS results for the ternary Au/Ag/AgCl NCs when we used the annealing treatment temperature at 100, 200, and 300°C because of low oxygen content in the ternary Au/Ag/AgCl NCs. Further increasing the heat treatment temperature up to 400°C, despite the ternary morphology could be basically maintained, obvious oxygen element will be observed from the EDS results as shown in Fig. S3, which is readily to form the oxygen isolated layer and results in negative influence on their following SERS performances. Meanwhile, the ultraviolet-visible-near-infrared (UV-vis-NIR) absorbance spectra of these different nano-samples including AgCl NPs, binary Ag/AgCl NPs, ternary Au/Ag/AgCl NPs and ternary Au/Ag/AgCl NCs unannealed and annealed target at 300°C were investigated to evaluate their optical properties, as seen in Fig. 4(d). Compared with AgCl, the obtained binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs show distinct LSPR peaks at a wavelength of ∼410 and ∼633 nm, respectively. In contrast, we pleasantly observed that the ternary Au/Ag/AgCl NCs displayed a remarkably enhanced light response across a broad spectrum range from 400 nm to 1100 nm, indicating that the unique cluster-like nanostructure played a pivotal role in broadening absorbance capacity. Moreover, the resultant Au/Ag/AgCl NCs annealed at 300°C can still further enhance broad-band light absorption property, which offers a greater potential for improving diverse photo-electronic applications under multiple excitation wavelengths.
Fig. 4. (a) The dispersion photograph of as-prepared AgCl NPs, binary Ag/AgCl NPs, ternary Au/Ag/AgCl NPs and ternary Au/Ag/AgCl NCs in water and a photograph of the as-obtained ternary Au/Ag/AgCl NCs annealed at 300°C powder (inset). (b) SEM and high-magnification SEM images (inset) of ternary Au/Ag/AgCl NCs annealed at 300°C. (c) HRTEM image of abundant surface defects formed in the ternary Au/Ag/AgCl NCs annealed at 300°C. (d) UV-vis-NIR absorption spectra of AgCl NPs, binary Ag/AgCl NPs, ternary Au/Ag/AgCl NPs and ternary Au/Ag/AgCl NCs unannealed and annealed at 300°C, respectively.
The excellent SERS performances of the as-synthesized ternary Au/Ag/AgCl NCs were firstly evaluated by using Rhodamine 6 G (R6G) as probe molecules under 633 nm excitation. For comparison, AgCl NPs, binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs were separately selected as reference SERS nano-substrates. Figure 5(a) shows SERS spectra of R6G molecules with concentration of 10−7 M adsorbed on the above four different samples. In the absence of plasmonic metallic particles, the Raman spectral lines of R6G molecules are too weak to be clearly detected on the bare AgCl NPs. In contrast, the dominating characteristic bands of R6G molecules in the range of 600∼1800cm-1 are all clearly detected in SERS spectra when binary Ag/AgCl NPs, ternary Au/Ag/AgCl NPs and Au/Ag/AgCl NCs are applied as substrates because of the formation SERS active sites based on the introduction of plasmonic metallic components. Obviously, conventional SERS EM originated from interaction of a laser beam with plasmonic Au and Ag components plays the dominating role for improving SERS signals in comparison with the CM mechanism between a semiconductor AgCl and probe molecules [4–6]. More importantly, as shown in Fig. 5(b), it can be visually found that the SERS peak intensities of overall characteristic bands that collected from ternary Au/Ag/AgCl NCs are much higher than that of binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs. Taking the peak at 1363 cm-1 as an example, the SERS peak intensity of R6G molecules in the presence of ternary Au/Ag/AgCl NCs is about 4.4 and 2.3 times compared with that of binary Ag/AgCl NPs and ternary Au/Ag/AgCl NPs, respectively. It reveals that the synergistic effect among ternary nanocomposites as well as unique cluster-like nanostructure with longitudinal channels for photoexcited carrier transfer can further lead to the efficient SERS activity of ternary Au/Ag/AgCl NCs. Subsequently, we further investigated the effect of annealing treatment in the SERS performance of ternary Au/Ag/AgCl NCs by changing thermal temperature in the range of 100∼400°C. Figure 5(c) demonstrates that the SERS spectra of R6G molecules adsorbed on the treated Au/Ag/AgCl NCs after thermal annealing treatments at different high temperatures. Apparently, the SERS signals of R6G molecules can be significantly enhanced by increasing the thermal temperature, and Raman peak intensities reach the maximum values until the temperature up to 300°C. The SERS peak intensities will reduce with further elevating annealing temperature at 400°C, as the signal-evolution described in Fig. 5(c). Accordingly, Fig. 5(d) intuitively reflects the SERS peak intensities of R6G characteristic bands show a similar increasing trend with increasing the heat temperature to 300°C, and exhibit some reducing values at excess high temperature of 400°C. In fact, the thermal annealing treatment in the non-vacuum environment could not only improve the interaction among ternary components but also introduce abundant surface oxygen defects into the ternary Au/Ag/AgCl NCs, thereby accelerating the photoinduced electron transfer between these substrates and molecules and further boosting the SERS activity. As a result, stronger interaction among ternary components and increasing abundant surface oxygen defects could contribute to the enhanced SERS activity when the annealing treatment temperature changes from 100 to 300°C. Further increasing the annealing temperature up to 400°C, the similar oxygen isolated layer will occur in the ternary Au/Ag/AgCl NCs with the introduction of excess oxygen, which will hinder the contact of ternary Au/Ag/AgCl NCs and target molecules and then decrease the SERS activity compared with that at 300°C annealing. There are about more than 3-fold SERS enhancement for all characteristic peak intensities of R6G molecules by adopting the optimal 300°C thermal annealing treatment, in comparison with the pristine one. These results imply that the promoted SERS activity of the resultant Au/Ag/AgCl NCs is highly related to the moderate annealing treatment at appropriate high temperature of 300°C that should give rise to the surface modification of the hybrid nano-substrates and then enable the corresponding electronic structures of ternary components to be changed in this work.
Fig. 5. (a) SERS spectra of 10−7 M R6G adsorbed on AgCl NPs, binary Ag/AgCl NPs, ternary Au/Ag/AgCl NPs and Au/Ag/AgCl NCs, respectively. (b) The intensity variation of R6G characteristic peaks at the above four different SERS substrates. (c) SERS spectra of 10−7 M R6G adsorbed on ternary Au/Ag/AgCl NCs substrates annealed at different temperatures. (d) The intensity variation of R6G characteristic peaks at different annealed temperatures.
In order to evaluate the thermal annealing-induced surface modification of the resultant Au/Ag/AgCl NCs, X-ray photoelectron spectroscopy (XPS) analyses of the same sample before and after annealing treatment at 300°C were investigated in this work, as represented in Fig. 6. The XPS survey scans shown in Fig. 6(a) display clearly observable characteristic peaks of Au, Ag and Cl species. Besides the inherent C and O elements in the routine XPS measurement, no obvious XPS signals from any other impurities could be found for the target before and after heat treatment at 300°C. It reveals the high-purity of Au/Ag/AgCl NCs with ultra-clean surfaces are conveniently generated by the green laser-induced synthesis and then modified by thermal annealing treatment. As displayed in Fig. 6(b), the high-resolution XPS pattern of Cl 2p originated from unannealed Au/Ag/AgCl NCs can be deconvoluted into two peaks at 197.90 and 199.52 eV, which should be severally assigned to the characteristic doublets of Cl 2p3/2 and Cl 2p1/2 [44–46]. As for the modified Au/Ag/AgCl NCs, the doublet peaks of Cl 2p are separately measured at 198.32 and 199.94 eV, implying a positive shift of 0.42 eV in comparison with the original sample. On the other hand, the obvious change of Ag 3d electronic structure originated from two samples is also verified in Fig. 6(c). The doublet XPS peaks of Ag 3d derived from the unannealed Au/Ag/AgCl NCs are detected at 367.61 and 373.60 eV, which should be attributed to the binding energy of Ag 3d5/2 and Ag 3d3/2 that can be deconvoluted into four peaks, respectively [44–46]. The XPS peaks located at 367.61 and 373.60 eV were ascribed Ag+ (AgCl), and the two XPS peaks at 368.30 and 374.30 eV can be ascribed to metallic Ag°. Compared to unannealed ternary Au/Ag/AgCl NCs, the four XPS peaks of Ag 3d originated from the treated Au/Ag/AgCl NCs after thermal annealing at 300°C are already shifted to higher binding energies with redshift of 0.45 eV. Meanwhile, the XPS peaks of Au 4f can be also deconvoluted into two peaks (Fig. 6(d)) at 83.98 and 87.67 eV, which can be attributed to the formation of Au° in the obtained ternary Au/Ag/AgCl nanoproducts. Similarly, the further positive shift of 0.35 eV derived from the binding energy of Au 4f can be also observed on the treated Au/Ag/AgCl NCs in comparison with pristine one, as illustrated in Fig. 6(d). Interestingly, O 1s attributed to the surface oxygen adsorption occurred in typical XPS measurements [47] exhibits a negative shift of 0.74 eV before and after annealing treatment at 300°C as shown in Fig. S4. Evidently, compared with unannealed ternary nanoproducts, the continuous positive shifts of XPS characteristic peaks originated from Cl, Ag and Au species, indicating the formation of more positive charges in the ternary Au/Ag/AgCl NCs after annealing at 300°C because of the easier electrons transfer from Au/Ag/AgCl NCs to surface oxygen [48]. As a result, an even stronger interaction among ternary components would exist after the appropriate thermal annealing treatment. It is particularly beneficial to enhance the synergistic effect of the resultant ternary nano-composites, accelerating the photoinduced electron transfer between these substrates and molecules [49,50] and thereby further boosting the SERS activity.
Fig. 6. XPS spectra of the prepared ternary Au/Ag/AgCl NCs unannealed and annealed at 300°C, respectively. (a) Survey spectra, (b) Cl 2p, (c) Ag 3d, (d) Au 4f.
To assess the potential SERS detection of the resultant ternary Au/Ag/AgCl NCs, we measured the detection limit of R6G molecules on the prepared nano-substrates. Figure 7(a) shows all SERS spectra of R6G molecules with concentration decreased from 10−7 to 10−11 M under 633 nm excitation. The characteristic peaks at 613, 773, 1313, 1363, 1512 and 1651 cm−1 can be distinctly detected even when the concentration of R6G molecules is dropped to 10−11 M, indicating that the obtained SERS nano-substrate has a higher sensitivity at low molecular concentration. Moreover, the good linear relationships for the plots of SERS peak intensities of characteristic bands at 613, 773, 1313, 1363, 1512 and 1651 cm−1 versus the concentrations of R6G molecules are illustrated in Fig. 7(b). It shows that the resultant Au/Ag/AgCl NCs-based SERS nano-substrate is a potential candidate for the quantitative and quick detection of dye molecules in a wide range of concentrations. To further verify the spatial SERS uniformity of the resultant Au/Ag/AgCl NCs, based on the 30 even-distributed measurement points in a randomly chosen area of the target, the corresponding Raman spectra are displayed in Fig. S5. Clearly, the Raman signals of probe molecules can be well repeated at random points on the planar surface, implying the excellent SERS signal homogeneity of the prepared nano-substrate. In detail, the variations of SERS peak intensities of characteristic bands at 613, 773 and 1651 cm−1 versus different random 30 points are illustrated in Fig. 7(c). The corresponding relative standard deviation (RSD) values of these three characteristic peak intensities are calculated to be about 4.02%, 4.48% and 3.12%, respectively, supporting the reliable spatial SERS uniformity of the resultant Au/Ag/AgCl NCs.
Fig. 7. (a) SERS spectra of R6G molecules with different concentrations adsorbed on the resultant ternary Au/Ag/AgCl NCs. (b) The variations of SERS intensities (the height of the signals) versus the concentration of R6G molecules. (c) The SERS intensity distribution at 613, 773 and 1651 cm-1 of 10−9 M R6G from randomly thirty measured sites on ternary Au/Ag/AgCl NCs substrate (the average intensity is indicated by black line, and the green zones represent the corresponding intensity variation).
Considering that the resultant ternary Au/Ag/AgCl NCs possess broadband light response across a broad spectrum range from 400 nm to 1100 nm, we employed four laser wavelengths at 457, 532, 633, 785 and 1064 nm to excite the interesting SERS substrate for evaluating its multi-resonant SERS performance. The different excitation wavelengths-triggered SERS spectra of 10−7 M R6G molecules adsorbed on the resultant Au/Ag/AgCl NCs and reference sample composed of monodisperse Au/Ag/AgCl NPs are shown in Fig. 8. Initially, the reference Au/Ag/AgCl NPs show discernible SERS characteristic peaks of probe molecules under 633 nm laser excitation, very weak Raman signals under 457, 532, 785 or 1064 nm laser irradiation in Fig. 8(a). The limited narrowband SERS of monodisperse Au/Ag/AgCl NPs-based reference sample is exactly consistent with its single-optical resonant condition composed of bare strong LSPR peak appeared at ∼633 nm (Fig. 4(d)). In sharp contrast, the overall SERS signal intensities of R6G molecules adsorbed on the resultant Au/Ag/AgCl NCs are obviously enhanced by using all the four visible-NIR light wavelengths as displayed in Fig. 8(b), in comparison with that of reference sample in Fig. 8(a). For instance, the SERS peak intensity of characteristic band at 1363 cm-1 derived from the resultant Au/Ag/AgCl NCs under 532 nm laser excitation is measured about 15155 a.u that is already about 44.1 times higher than that of the reference Au/Ag/AgCl NPs. Additionally, compared with those under 457, 532, 633 and 785 nm excitation, the overall SERS intensity originated from the resultant Au/Ag/AgCl NCs is obviously reduced under laser excitations at 1064 nm due to its lower light conversion efficiency, which is related to the resonance peak of Au or Ag components [51,52]. However, the SERS characteristic peaks of R6G molecules employing ternary Au/Ag/AgCl NCs as substrates can be still clearly distinguished excited at 1064 nm in comparison to that using ternary NPs SERS substrates. In conclusion, it reveals that the improved broadband SERS performance under broadband visible-NIR laser excitations can be successfully realized on the resultant ternary Au/Ag/AgCl NCs with excellent light response and then enhanced the synergistic effect by thermal annealing treatment. The resultant ternary Au/Ag/AgCl NCs can support an outstanding broadband SERS operation under laser excitations at 457, 532, 633, 785 and 1064 nm. Whereafter, a series of SERS spectra derived from 10−7 M R6G probe molecules adsorbed on the resultant ternary Au/Ag/AgCl NCs after freezing at cryogenic condition of -196°C were separately recorded by 457, 532, 633, 785 and 1064 nm laser excitations. As illustrated in Fig. S6, it is clearly found that the SERS characteristic peak intensities derived from the ternary nano-substrates by either 457, 532, 633, 785 or 1064 nm wavelength excitation can be obviously boosted via dropping the temperature to cryogenic condition at -196°C. As a result, the low temperature could be indeed regarded as an effective way to enhance the SERS activity of the ternary Au/Ag/AgCl NCs, which could be further verified by these five different incitation wavelengths including 457, 532, 633, 785 and 1064 nm. More importantly, much clearer characteristic peaks and good signal to noise ratio of R6G molecules could be observed adopting 633 nm as the excitation wavelength via the ternary Au/Ag/AgCl NCs as SERS substrates. In this way, the following SERS experiment were carried out under the 633 nm excitation that is more suitable for getting high-performance SERS under cryogenic condition.
Fig. 8. SERS spectra of 10−7 M R6G adsorbed on the ternary Au/Ag/AgCl NPs (a) and resultant ternary Au/Ag/AgCl NCs (b) with the 457, 532, 633, 785 and 1064 nm excitations, respectively.
In order to get a deep insight into the cryogenic temperature-boosted SERS activity of the resultant ternary Au/Ag/AgCl NCs, the temperature-dependent SERS spectra of R6G probe molecules are performed in Fig. 9(a). The results demonstrate that the SERS peak intensities of R6G molecules significantly increase with declining the temperature from normal 20°C to cryogenic -196°C. In detail, the variations of SERS characteristic peak intensities of probe molecules under different temperatures are shown in Fig. 9(b), and each group of SERS peak shows a very similar upward trend by decreasing the temperature to cryogenic condition in this work. Indeed, the overall SERS peak intensities displayed in Fig. 9(b) are already enhanced more than 5 times at low temperature -196°C compared to that at room temperature 20°C. Thus, the cryogenic temperature can be assuredly regarded as an effective technique for further boosting SERS activity of the resultant Au/Ag/AgCl NCs. To the best of our knowledge, the cryogenic temperature-enhanced SERS activity of the resultant Au/Ag/AgCl NCs should be highly related to the increased PICT efficiency that can be simply verified by PL emission spectroscopy [19–23]. Then, the temperature-dependent PL emission spectroscopies of the prepared sample excited by an excitation wavelength of 633 nm were carefully performed as displayed in Fig. 9(c). Obviously, the prominent PL peak of the resultant Au/Ag/AgCl NCs is located at ∼685 nm originated from the radiative decay process, which is dramatically enhanced with the decrease of temperature from normal 20°C to cryogenic condition at -196°C. In detail, the PL peak intensity recorded at low temperature -196°C is obviously increased by ∼5.4 fold as compared to that at room temperature 20°C in Fig. 9(d). The increased PL intensity of the prepared sample in Fig. 9(c)-9(d) reveals that the number of photoinduced electrons can be increased at cryogenic temperature. Moreover, the detailed mechanism of the enhanced SERS activity by boosting PICT efficiency at cryogenic temperature is illustrated in this work (Supplement 1). As a result, the low temperature can be safely regarded as an effective strategy to weaken the lattice thermal vibration and suppress phonon-assisted relaxation [19–23], thereby suppressing the recombination of photon-triggered electrons and lattice vibration-induced thermal phonons to essentially improve the PICT efficiency and then enhance SERS activity. Significantly, compared with the conventional chemical synthesis protocols, the projection using laser light as a versatile and convenient tool for sculpting novel “green” ternary Au/Ag/AgCl NCs can avoid the introduction of external complicated additives that is hard to be removed even suffering from multistep yet tedious operations [53–55]. Especially, most of these residual polymer molecules on nanomaterials will be condensed at low temperature condition, which significantly hinders the electron transfer on the substrate-molecule interface and then reduces PICT transition seriously. Therefore, the clean surface of ternary Au/Ag/AgCl NCs will avoid low temperature condensation and make them to directly and effectively contact with probe molecules under cryogenic condition. As a result, the obtained ternary Au/Ag/AgCl NCs with ultra-clean surface via laser irradiation could be considered as promising substrates for the subsequent research on cryogenic SERS performance.
Fig. 9. (a) Temperature-dependent SERS spectra of 10−7 M R6G adsorbed on the resultant ternary Au/Ag/AgCl NCs. (b) Histogram of SERS characteristic peaks of R6G molecules at different temperatures. (c) PL emission spectra of the resultant ternary Au/Ag/AgCl NCs at different temperatures with the 633 nm excitations. (d) Histogram of PL peak intensity at different temperatures.
To further evaluate the cryogenic temperature-triggered SERS detection capability of the resultant ternary Au/Ag/AgCl NCs, the SERS spectra of R6G probe molecules with different concentrations ranging from 10−7 to 10−15 M separately adsorbed on the prepared nano-substrate were recorded at low temperature of -196°C. As described in Fig. 10(a), the resultant Au/Ag/AgCl NCs exhibit remarkably enhanced SERS sensitivity with a 10−15 M limit detection for R6G molecule at low temperature -196°C. The LOD value obtained at cryogenic condition is already achieved at ultra-trace femtomole (fM) level, which is significantly better than that of 10−11 M obtained at room temperature 20°C (Fig. 7(a)). Besides, there are also some well-defined good linear relationships for the plots of SERS peak intensities of characteristic bands at 613, 773, 1313, 1363, 1512 and 1651 cm−1 versus the concentration of R6G molecules, as illustrated in Fig. 10(b). It shows that the resultant Au/Ag/AgCl NCs-based SERS nano-substrate at cryogenic condition is still a potential candidate for the quantitative and quick detection of probe molecules at ultralow concentration. Moreover, in order to confirm the spatial SERS uniformity of the resultant Au/Ag/AgCl NCs at cryogenic temperature, the mapping columns of SERS characteristic signals of R6G molecules are also collected at random 30 different points on the prepared target. In combination with optical images (Fig. S7), the spacing between the two points must be greater than 0.5 mm in order to ensure that the selected points represent the uniformity of the entire sample, considering the size of the SERS substrate (3.5 mm × 3.5 mm). As shown in Fig. S8, the repeatable SERS signals with similar bright fringe in each mapping column at random spatial points can verify the excellent SERS uniformity at cryogenic temperature. Based on the variations of SERS characteristic peak intensities at 613, 773 and 1651 cm-1 versus different 30 random points illustrated in Fig. 10(c), the corresponding RSD values are calculated about 2.79%, 3.51% and 1.71%, respectively. The calculated results further indicate that the excellent SERS uniformity of the resultant ternary Au/Ag/AgCl NCs can be maintained by declining the temperature to cryogenic condition at -196°C.
Fig. 10. (a) SERS spectra of R6G molecules with different concentrations adsorbed on the resultant ternary Au/Ag/AgCl NCs at -196°C. (b) The variations of SERS intensities (the height of the signals) versus the concentration of R6G molecules. (c) Based on 10−11 M R6G molecules, the variations of Raman peak intensities at 613, 773 and 1651 cm-1 versus random 30 points on the resultant ternary Au/Ag/AgCl NCs (the average intensity is indicated by black line, and the gray zones represent the corresponding intensity variation).
Significantly, the cryogenic temperature-triggered high-performance SERS of the resultant ternary Au/Ag/AgCl NCs was finally extended to the ultrasensitive detection of folic acid (FA) biomolecules at trace concentration. As a typical cell-targeting agent, FA has great value for various biochemical processes such as the formation and maturation of blood cells and cell multiplication [56,57]. As illustrated in Fig. 11(a)–11(b), the SERS spectra of FA biomolecules with different concentrations are collected from the resultant Au/Ag/AgCl NCs at room temperature 20°C and cryogenic temperature -196°C, respectively. Clearly, the SERS spectra exhibit all the characteristic bands of FA biomolecules detected at 688, 1178, 1504 and 1589 cm−1 that are consistent with the previous reports [56,57], supporting the excellent spectroscopic distinguishability and practicability of the resultant Au/Ag/AgCl NCs toward practical application. Moreover, it can be found that the LOD of FA biomolecules at normal temperature can be achieved at concentration of 10−9 M in Fig. 11(a). Compared with that at room temperature of 20°C, the prominent characteristic bands of FA biomolecules can be clearly identified even at an ultralow concentration of 10−12 M by adopting low temperature at -196°C in Fig. 11(b). Obviously, the cryogenic temperature-improved LOD of FA biomolecules at 10−12 M in this work certainly exceeds many other previous works similarly adopting metal-based nanostructures as highly active SERS substrates [58–61]. Additionally, the SERS peak intensities of characteristic bands at 1178 and 1589 cm−1 were positively correlated to the FA concentration at range of 10−12 M to 10−8 M (Fig. 11(c)). Accordingly, the corresponding linear equations were displayed as y = 1639.31x + 20034.18 (R2 = 0.987) and y = 3252.42x + 39643.02 (R2 = 0.982), respectively. These results indicate that the proposed Au/Ag/AgCl NCs display good potential for acting as a pronounced SERS-based biosensor toward the reliable and precious assessment of FA biomolecules in practical scenes. Based on the Raman scanning model, the mapping diagrams of SERS peak intensities at 688, 1178, 1504 and 1589 cm−1 performed on micro-scale areas are displayed in Fig. 11(d), revealing that there no obvious color-difference for each typical characteristic peak at different points. More importantly, the RSD values of the SERS mapping diagrams at 688, 1178, 1504 and 1589 cm−1 are separately calculated to be 4.83%, 4.58%, 4.92% and 4.27%, respectively, confirming the reliable SERS signal homogeneity on the resultant Au/Ag/AgCl NCs at low temperature condition. In conclusion, the resultant ternary Au/Ag/AgCl NCs can be regarded as an attractive SERS biosensor for cryogenic temperature-assisted high-performance SERS application, which will provide a new approach for further enhancing the SERS activity of metal/semiconductor-based nano-substrates.
Fig. 11. (a) SERS spectra of FA molecules with different concentrations adsorbed on the resultant ternary Au/Ag/AgCl NCs at 20°C. (b) SERS spectra of FA molecules with different concentrations adsorbed on the resultant ternary Au/Ag/AgCl NCs at -196°C. (c) The variations of SERS intensities (the height of the signals) versus the concentration of FA molecules. (d) The micro-scale mapping diagrams of Raman peak intensities at 688, 1178, 1504 and 1589 cm-1, respectively.
4. Conclusions
In summary, an excellent and reliable SERS substrate with high sensitivity and broadband response property based on the ultra-clean ternary Au/Ag/AgCl NCs was successfully demonstrated in this report. Typically, the ternary Au/Ag/AgCl NCs with clean surface were synthesized one step by simply adopting 532 nm pulsed laser irradiation of the mixed AgNO3 and HAuCl4 solution. Interestingly, compared with the reference ternary NPs, the resultant ternary Au/Ag/AgCl NCs with unique 1D cluster-like morphology exhibit an obvious wideband absorption in the wavelength range of 400∼1100 nm, and remarkable broadband SERS performance was observed employing 457, 532, 633, 785 and 1064 nm laser excitation. Especially at the excitation wavelength of 633 nm, owing to the improved surface defects and interaction among the ternary components, the resultant ternary Au/Ag/AgCl NCs substrates annealed at 300°C exhibit more than 3-fold higher SERS signals than unannealed sample. More importantly, compared with that at room temperature 20°C, the SERS activity of the resultant ternary Au/Ag/AgCl NCs can further amplify more than 5 times at cryogenic -196°C and allow the highly sensitive detection of FA at low concentration of 10−12 M attributed to the obviously increased PICT transition at cryogenic condition. This work provides a fundamental reference for cryogenic temperature-triggered ultrahigh multiple SERS activities under visible-NIR excitations of ternary Au/Ag/AgCl NCs and expand its application on cryogenic sensing and hypothermal medicine.
Funding
Natural Science Foundation of Shandong Province (ZR2021QA064, ZR2023QB283, ZR2023QF157); Fundamental Research Fund of Shandong University (2018JC022); Shandong Jianzhu University (1608); National Natural Science Foundation of China (11575102, 11905115).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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