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Article

Nanoporous Ag-Decorated Ag7O8NO3 Micro-Pyramids for Sensitive Surface-Enhanced Raman Scattering Detection

by
Linfan Guo
1,
Haibin Tang
2,
Xiujuan Wang
3,
Yupeng Yuan
1 and
Chuhong Zhu
1,*
1
School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, Anhui University, Hefei 230601, China
2
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China
3
School of Microelectronics, Hefei University of Technology, Hefei 230009, China
*
Author to whom correspondence should be addressed.
Chemosensors 2022, 10(12), 539; https://doi.org/10.3390/chemosensors10120539
Submission received: 25 October 2022 / Revised: 12 December 2022 / Accepted: 14 December 2022 / Published: 16 December 2022
(This article belongs to the Special Issue Nanocomposites for SERS Sensing)
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Abstract

:
Porous noble metal nanomaterials can be employed to construct sensitive surface-enhanced Raman scattering (SERS) substrates, because the plasmonic nanopores and nanogaps of the porous materials can provide a larger number of hotspots, and can also serve as containers of analyte molecules. However, the fabrication processes of nanoporous noble metal are generally complicated. Here, a facile method is presented to prepare nanoporous Ag nanoparticles-decorated Ag7O8NO3 micro-pyramids, which are fabricated through the chemical reduction of the electrodeposited Ag7O8NO3 micro-pyramids using NaBH4. The Ag7O8NO3 micro-pyramids are fabricated by electrodeposition by using a simple aqueous solution of AgNO3 as electrolyte. Then, porous Ag-decorated Ag7O8NO3 micro-pyramids are achieved by the chemical reduction of the surface of the electrodeposited Ag7O8NO3 micro-pyramids with NaBH4. The high-density nanopores and nanogaps of the fabricated nanoporous Ag can provide plenty of hot spots for Raman enhancement. Additionally, the nanopores have an effective capacity to trap and enrich analytes. Using rhodamine 6G (R6G) as a probe molecule, the SERS performance of the fabricated SERS substrate has been investigated. It is found that a limit of detection (LOD) ~1.0 × 10−15 M can be achieved for R6G. Then, the SERS substrates are employed to detect dye molecule (crystal violet) and pesticide (thiram), and their LODs are calculated down to 9.6 × 10−13 M and 1.3 × 10−15 M, respectively. The enhancement factor of the fabricated SERS substrate is estimated to be as high as 5.6 × 108. Therefore, the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids have shown promising application in the sensitive SERS detection of organic molecules.

1. Introduction

Surface-enhanced Raman scattering (SERS) spectra can provide fingerprint information of molecular vibration with high sensitivity. Therefore, SERS can serve as an effective and convenient analytical tool, and has been utilized substantially in the rapid, sensitive, and nondestructive detection of chemical and biological agents [1,2,3,4,5,6,7,8]. A major driving force for achieving sensitive response to a substance’s adsorption on a metal surface is localized surface plasmon resonance (LSPR). In particular, the collective plasmon resonance arising from the LSPR interaction of Ag and Au [9,10] is utilized by SERS. However, LSPR energies are dependent on the local dielectric environment, the size, shape, and composition of these metal nanostructures [11,12,13,14]. The methods for creating noble metal nanoparticles with sharp tips [7,14,15,16] or nano-gaps [17,18], as well as nanostars [19,20], nanoflowers [21,22], and porous nanoparticles [23,24,25], have been the subject of studies in recent years. For example, highly ordered arrays of solid gold and silver nanopyramids have been developed for reproducible and sensitive SERS detection [26,27,28,29]. Porous nanostructures have garnered a lot of attention as compared to other structures (e.g., the above-mentioned arrays of solid gold and silver nanopyramids) because of their enormous relative surface area, high internal “hot spot” density, and optical properties linked to porosity [30,31,32]. In addition to having a high surface area, high gas permeability, and low density in a variety of porous materials, porous noble metal materials also possess surface plasmon resonance properties that have numerous uses in the optical field [32,33,34,35]. The fascinating networks of interconnected structures that make up the nanoporous metal structures have a morphology with tunable open pores and ligament sizes ranging from nanometers to micrometers, producing a significant amount of specific surface area [36,37]. Porous metal nanomaterials have so far been prepared using a variety of physical and chemical techniques, such as the template method [38,39,40,41], galvanic replacement [42,43,44], and dealloying method [45,46,47]. These methods make the preparation process extra intricate and expensive. Therefore, there is still a critical need to develop a facile and inexpensive approach for fabrication of nanoporous plasmonic structures with the capability to tailor the structural parameters for optimized performance.
Here, we present a rapid and cost-effective method to fabricate nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. By chemically reducing the Ag7O8NO3 micro-pyramids, which had been prepared in advance by electrodeposition, nanoporous Ag-decorated Ag7O8NO3 micro-pyramids with dense nanopores have been fabricated without using any organic chemicals or etching agents. As shown in Scheme 1a, the Ag7O8NO3 micro-pyramids are firstly formed by electrodeposition. Then, the as-prepared Ag7O8NO3 micro-pyramids are immersed in an aqueous solution of NaBH4 (Scheme 1b). Using NaBH4 as a reducing agent, nanoporous Ag-decorated Ag7O8NO3 micro-pyramids are achieved by optimizing experimental conditions (Scheme 1c). Furthermore, we used the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids as SERS substrate to detect chemical molecules such as R6G, crystal violet (CV), and thiram. The nanoporous Ag-decorated Ag7O8NO3 micro-pyramids showed excellent SERS performance with the capability to sensitively detect and identify organic pollutants. Therefore, the fabricated nanoporous Ag-decorated Ag7O8NO3micro-pyramids have promising applications in SERS-based detection.

2. Experimental

2.1. Materials

Silver nitrate (AgNO3) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium borohydride (NaBH4), Rhodamine 6G (C28H31ClN2O3), crystal violet (C25H30ClN3), and thiram (C6HN2S4) were purchased from Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Indium tin oxide (ITO) glasses were bought from Huananxiangcheng Technology Co., Ltd. (Shenzhen, China). Deionized water (18.25 MΩ cm−1) was obtained from a Millipore water purification.

2.2. Electrochemical Fabrication of Ag7O8NO3 Micro-Pyramids

Electrochemical deposition was carried out using a two-electrode system, as shown schematically in Scheme 1a. A rectangular graphite sheet was used as an anode, an ITO glass (2 cm × 0.5 cm) was used as a cathode, and an aqueous solution of AgNO3 (0.01 M) was employed as the electrolyte. At room temperature, the potentiostatic mode was applied for the electrodeposition of Ag7O8NO3 micro-pyramids. By regulating electrodeposition voltage from 1.5 V to 10 V or deposition duration from 1 s to 300 s, Ag7O8NO3 materials with different micro-structures were achieved. Then the ITO glass coated with the as-prepared Ag7O8NO3 micro-pyramids was taken out, rinsed with deionized water, and dried by flowing Ar gas.

2.3. Formation of Nanoporous Ag-Decorated Ag7O8NO3 Micro-Pyramids

An ITO glass covered with Ag7O8NO3 micro-pyramids was vertically immersed into an aqueous solution of NaBH4 (10 mL) with different concentrations for 2 min to reduce silver ion on the surface of the Ag7O8NO3 micro-pyramids into Ag atoms for formation of nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. After the reduction process, the ITO glass coated with the fabricated nanoporous Ag-decorated Ag7O8NO3 micro-pyramids was taken out, rinsed with deionized water, and dried by flowing Ar gas.

2.4. Characterization and SERS Measurements

The resultant compact Ag7O8NO3 micro-pyramids and nanoporous Ag-decorated Ag7O8NO3 micro-pyramids were characterized by using X-ray diffraction (XRD, Rigaku SmartLab, Beijing Co, Beijing, China), X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Waltham, MA, USA) scanning electron microscope (SEM, Hitachi S-4800, Hitachi, Tokyo, Japan), and transmission electron microscope (TEM, JEM-2100F, JEOL, Tokyo, Japan). SERS measurement was conducted on a confocal microprobe Raman system (inVia, Renishaw, London, UK) with a 532 nm excitation laser. The laser power was ~1 mW, and the integration duration was 10 s.

2.5. Preparation of SERS Sample

To ensure good adsorption of analyte molecule, nanoporous Ag-decorated Ag7O8NO3 micro-pyramids on an ITO glass (0.5 cm × 0.5 cm, Figure S1) were immersed in an aqueous solution (10 mL) of R6G, CV or an ethanol solution (10 mL) of thiram with different concentrations for 2 h, then taken out and dried with flowing Ar gas. During SERS measurement, 10 different points were randomly selected on each substrate to analyze the SERS-spectral reproducibility of the substrate.

3. Results and Discussion

3.1. Characteristics of Ag7O8NO3 Micro-Pyramids

Ag7O8NO3 micro-pyramids are firstly fabricated by electrodeposition in a simple aqueous electrolyte of AgNO3. All the diffraction peaks observed in the X-ray diffraction pattern (XRD) of the as-prepared sample (Figure S2) correspond to an Ag7O8NO3 crystal (JCPDS: 06-0434); these diffraction peaks are assigned to the (2 2 2), (4 0 0), (4 4 0), and (6 2 2) planes, indicating that Ag7O8NO3 micro-pyramids have been successfully prepared [48]. The EDX mapping was conducted to confirm Ag and O distributions of Ag7O8NO3 micro-pyramids (Figure S3), which are consistent with those reported previously [48]. The morphology of the fabricated Ag7O8NO3 micro-pyramids could be regulated by tuning the deposition voltage and deposition duration [49,50,51,52]. Firstly, the morphological evolution of the pyramids was investigated by gradually increasing the deposition voltage from 1.5 to 10 V while the deposition time was kept unchanged (Figure S4). Sparsely distributed Ag7O8NO3 micro-pyramids were preliminarily formed at 1.5 V with an average edge length of 650 nm (Figure S4a). With the increase of deposition voltage, the density of Ag7O8NO3 micro-pyramids was increased accordingly (Figure S4b–d). Under a deposition voltage of 3 V, pyramids with an average edge of ~1.5 μm were formed and uniformly distributed on an ITO glass (Figure S4b). Nevertheless, with further increase of deposition voltage from 3 V to 5 V or even 10 V, arrowhead-like particles with a smaller size were achieved (Figure S4c,d), due to the presence of the electrocarving process at high deposition voltages. The change in pyramid size can be explained by the fact that more Ag7O8NO3 nuclei produced during the same deposition time at higher deposition voltages (>5 V). High deposition voltages produce more Ag7O8NO3 species than low deposition voltages, but because of the large number of nuclei, only a limited number of Ag7O8NO3 species that can be consumed at once. As a result, smaller Ag7O8NO3 pyramids are formed at high deposition voltages. This is can proved by the number density of Ag7O8NO3 pyramid (Figure S4c,d), which increases quickly as the deposition voltage enhanced [48]. In this work, we focus on the pyramid-like structures, therefore the voltage used for deposition was set as 3 V in the following experiments.
Then, we further studied the growth process of the pyramids under different duration of electrodeposition by keeping the deposition voltage (3 V) unchanged (Figure S5). Quasi-pyramid-like nanoparticles with a rough surface were formed in a very short time (1 s) (Figure S5a). As the deposition duration increased, more regular pyramid structures with larger size were gradually formed (Figure S5b–d). Larger plasmonic particle has a larger surface area, which can provide more adsorption sites for analyte molecules [53,54]. Therefore, the Ag7O8NO3 micro-pyramids with largest size were used to fabricate SERS-active structures since they can provide more hotspots in the excitation laser beam for SERS detection. Hence, the samples achieved under the voltage of 3 V and electrodeposition duration of 300 s were employed for SERS detection in the following experiments.

3.2. Fabrication of Nanoporous Ag-Decorated Ag7O8NO3 Micro-Pyramids

Figure 1a shows the scanning electron microscope (SEM) image of the as-prepared Ag7O8NO3 micro-pyramids. The Ag7O8NO3 micro-pyramids have a smooth surface with an average edge length of ~3.2 μm. It can be found that the electrodeposited Ag7O8NO3 micro-structure tends to form an octahedral shape. After fabrication of Ag7O8NO3 micro-pyramids, we decorate SERS-active material on their surfaces by an in situ reduction method [21,55]. After being reduced by NaBH4, porous Ag was formed and uniformly modified on the surface of each Ag7O8NO3 micro-pyramid (Figure 1b). The reduced Ag was uniformly assembled on the surface of Ag7O8NO3 micro-pyramids and formed a nanoporous Ag shell coated on the micro-pyramids (Figure 1b). Transmission electron microscopy (TEM) observation indicates that the Ag nanostructures on the Ag7O8NO3 micro-pyramids are well crystallized (Figure 1c). The spacing between adjacent lattice fringes was 0.237 nm for Ag (inset in Figure 1c), which agree well with the interplanar distance of the (1 1 1) plane of metallic Ag (JCPDS: 04-0783). The nanoporous Ag shell is composed of many cross-linked ellipsoidal and/or rod-shaped Ag nanostructures (Figure 1d,e). The EDX mapping was conducted to confirm the element distributions of nanoporous Ag-decorated Ag7O8NO3 micro-pyramids (Figure S6), which is consistent with that reported previously [48]. Then, X-ray diffraction spectrum measurement was further performed to investigate the crystal structure of the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids (Figure 1f). The XRD pattern confirms that the sample are composed of two crystallized components: Ag and Ag7O8NO3. The XRD spectrum display the peaks around 38.1°, 44.3° and 64.4° correspond to the diffractions from the (1 1 1), (2 0 0) and (2 2 0) planes of the face-centered-cubic (fcc) phase, respectively (upper curve in Figure 1f), confirming that the resultant sample contains abundant Ag crystals. These results demonstrate that nanoporous Ag has been successfully decorated on the surfaces of Ag7O8NO3 micro-pyramids by NaBH4 reduction. Such nanoporous Ag-decorated Ag7O8NO3 micro-pyramids can be employed as three-dimensional SERS substrates for sensitive detection [13,14].
During the process of decorating nanoporous Ag on the Ag7O8NO3 micro-pyramids, the concentration of the reducing agent (i.e., NaBH4) plays an important role on the morphology of the reduction-produced Ag nanostructures. When the concentration of NaBH4 is very low, only a few Ag nanoparticles were anchored on the surface of the Ag7O8NO3 micro-pyramids (Figure 2a) after the Ag7O8NO3 micro-pyramids being immersed in an aqueous solution of NaBH4 (1 mM) for 2 min. When the concentration of NaBH4 increased to a moderate level (e.g., 5 mM), a larger amount of Ag plenty of nanopores were decorated on the surfaces of the Ag7O8NO3pyramids (Figure 2b). If the NaBH4 concentration continued to increase to a much higher value (e.g., 10 mM or 25 mM), the Ag nano-ligaments constantly cumulated, and the number of pores in the Ag nanofilm decreased remarkably (Figure 2c,d). The disappearance of nanometer-sized ligaments and pores would decrease the number of hotspots, and thus lower the SERS activity of the fabricated hybrid micro/nanostructures. To optimize SERS performance, the R6G (10−6 M) was used as a probe molecule for Raman measurement. The SERS activity of the Ag-decorated Ag7O8NO3 micro-pyramids achieved under different concentrations of NaBH4 solution changed obviously (Figure S7a). In order to observe the variation trend of Raman intensity more intuitively, Raman intensities at 1648 cm−1 peak obtained by using different samples prepared under different concentrations of NaBH4 are shown in Figure S7b. It can be found that the optimized SERS performance was obtained at NaBH4 concentration of 5 mM. The silver ligaments of the resultant nanoporous Ag are composed of numerous irregular cross-linked ellipsoidal and/or rod-shaped Ag nanostructures (Figure S8a), and the pores and ligaments sizes of the nanoporous Ag-decorated Ag7O8NO3micro-pyramids were around 41.6 nm and 48.8 nm (Figure S8b,c), respectively. The high-density nanopores (with an average diameter of ~40 nm) and nanogaps in the fabricated nanoporous Ag can provide plenty of hot spots for Raman enhancement. Additionally, the nanopores have an effective capacity to trap and enrich analyte molecules, resulting in further improvement of SERS sensitivity [56,57,58]. Therefore, the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids reduced using 5 mM NaBH4 solution will be used as for sensitive SERS detection in the following tests.

3.3. SERS Performance

Sensitivity is an important criterion for the performance of SERS substrates. For exploring the SERS activity of the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids, a frequently used organic substance, R6G, was used as a probe molecule. In order to understand the origin of Raman signal enhancement, the SERS activities of Ag7O8NO3 micro-pyramids before and after being decorated with nanoporous Ag were measured. It is found that the Ag7O8NO3 micro-pyramids do not possess SERS activity (Curve II in Figure S9) and the nanoporous Ag decorated on the Ag7O8NO3 microstructures has remarkable SERS activity (Curve I in Figure S9). Therefore, the SERS activity is contributed by the nanoporous Ag. The most pronounced bands of R6G molecules appear at 612 (C-C-C ring in-plane vibration), 775 (C-C-C-ring out-of-plane bending), 1362 (aromatic C-C stretching), 1508 (aromatic C-C stretching), and 1648 (aromatic C-C stretching) cm−1 [59]. Additionally, a linear relationship between the logarithmic concentrations of R6G and the fingerprint peaks (1648 cm−1) was discovered (Figure 3b) as shown below:
l o g I = 5.49 + 0.27 l o g C
where I is the peak intensity of the SERS spectra of R6G, and C is the concentration of R6G. The linear fitting result shows that the R2 value of R6G is 0.983, which proves that the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids have the ability of quantitative detection of R6G. According to the Formula (1), the limit of detection (LOD) of R6G could be calculated. The SERS signal intensity at the LOD is equivalent to three times of the standard deviation of Raman signal intensity from the blank sample [60,61,62,63,64]. The LOD of R6G was calculated to be 10−15 M, demonstrating that the fabricated nanoporous Ag-decorated Ag7O8NO3 micro-pyramids substrate has excellent sensitivity for R6G. Compared with the LODs of R6G achieved by using the other SERS substrates reported in the previous literatures, our nanoporous Ag-decorated Ag7O8NO3 micro-pyramids have a comparable or superior LOD (Supplementary Table S1 [65,66,67,68,69,70,71,72]).
As a significant parameter for quantitatively comparing SERS performance, the enhancement factor (EF) of the nanoporous Ag-decorated Ag7O8NO3micro-pyramids was determined by computing the ratio of SERS to normal Raman scattering (NRS) of R6G (Figure S10) using the following expression:
EF = I SERS N Ref I Ref N SERS = I S E R S C R e f V R e f S S E R S I R e f C S E R S V S E R S S R e f
where ISERS and INRS correspond to the SERS and NRS intensities, respectively. For estimation of EF, a 10 μL 10−11 M and two 10 μL R6G ethanol solutions with concentration of 10−11 M and 10−2 M were dropped on the substrates (2 × 2 mm2) and polyvinyl chloride film (π × 12 mm2), respectively. The EF for the band at 1648 cm−1 was calculated to be 5.6 × 108 (detailed calculation process can be found in the first part of the Supplemental Material), revealing the high SERS activity of the fabricated SERS substrate.
In addition to SERS sensitivity, uniformity of Raman signals is another crucial indicator for SERS substrates. As shown in Figure 3c, SERS spectra were collected from 10 random points on the fabricated SERS substrate. It can be found that the characteristic spectral peaks’ intensities of R6G at 1648 cm−1 collected from 10 random points on the fabricated SERS substrate are very similar, and the RSD value was calculated to be 10.72% (Figure 3d). The point-to-point RSD of substrates that are eligible for quantitative analysis is typically thought to need to be less than 20% [73]. Thus, the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids have also shown outstanding uniformity for SERS measurements besides high sensitivity.
In order to ulteriorly demonstrate the practical applicability of the SERS substrate, crystal violet (CV) was chosen as another probe molecules. CV, an alkaline dye, is an excellent dye. It is widely used when dying leather, paper, and other materials. Additionally, reagent goods, pharmaceutical intermediates, and industry all make extensive use of CV. As dosage is increased, a substantial amount of CV is emitted into wastewater, causing visible color changes in water bodies in terms of chromaticity. It seriously harms people, animals, and plants, and this harm could be long-lasting and dangerous due to its carcinogenicity, mutagenicity, and reproductive toxicity [74,75]. Before it may be discharged, waste water that contains CV must be treated to meet regulations. Therefore, it is essential to monitor the concentration of them. The characteristic peaks of CV are clearly observed and can be attributed to phenyl ring breathing mode (914 cm−1), C-phenyl, C-H in-plane antisymmetric stretching (1176 cm−1), C-phenyl in-plane antisymmetric stretching (1585 cm−1 and 1618 cm−1) [76,77]. Additionally, the linear fitting result show that the R2 value are 0.99 (Figure 4b) for CV. The LOD were calculated down to 9.6 × 10−13 M. It could be observed that Raman signals of CV collected from different points on the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids are consistent (Figure S11a), and the RSD values are evaluated to be 10.55% (CV, 1618 cm−1) (Figure S11b). Those results indicate that the fabricated substrate has high SERS sensitivity and superior uniformity again.
The applicability of the SERS substrate was also tested towards the detection of pesticide (thiram). As a low toxicity fungicide, thiram is widely used in the elimination of agricultural pests. The relatively strong Raman peaks of thiram were assigned to S-S stretching mode (560 cm−1), CN stretching mode and the symmetric CH3 deformation mode (1384 cm−1), and CN stretching vibrations and CH3 rocking mode (1517 cm−1) [7,78]. The linear fitting result shows that the R2 value is 0.99 (Figure 4d). The LOD was calculated down to 1.3 × 10−15 M (3.12 × 10−10 ppm), which is below the maximal residue limit (MRL) of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency (EPA) [79,80,81]. Additionally, it was showed that relatively consistent Raman intensities were found in all SERS active points (Figure S11c). Through statistical calculation, the RSD of the Raman intensity at 1384 cm−1 was ~13.93% (Figure S11d). Such experimental results indicate that the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids can serve as effective SERS substrates for sensitive detection of pesticide molecules.

4. Conclusions

A facile method is presented to prepare nanoporous Ag nanoparticles-decorated Ag7O8NO3 micro-pyramids through the chemical reduction of the as-electrodeposited Ag7O8NO3 micro-pyramids using NaBH4. By regulating the concentration of NaBH4, the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids with highly accessible surfaces were obtained, and a larger number of Ag nanopores were formed on the surfaces of the Ag7O8NO3 pyramids. The nanoporous with an average diameter of 40 nm can serve as analyte containers, which efficiently trap and enrich analytes, and also provide high density SERS hotspots giving rise to outstanding SERS performance. For the analyte molecules such as rhodamine 6G, crystal violet and thiram, the fabricated nanoporous Ag-decorated Ag7O8NO3 micro-pyramids substrates can realize sensitive and uniform detection. The fabricated nanoporous Ag-decorated Ag7O8NO3 micro-pyramids have potential applications in the rapid, sensitive, and direct detection of dyes and pesticides, and the method may also be extended to fabrication of other nanoporous materials. Consequently, the easy and time-saving fabrication process and the outstanding SERS performance ensure the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids have promising applications in trace detection of harmful pollutants.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemosensors10120539/s1, 1. Estimation of enhancement factor. Figure S1: The fabricated nanoporous Ag-decorated Ag7O8NO3 micro-pyramids on ITO glass (2 cm × 0.5 cm) (left) and the actual substrate cut down for SERS detection (0.5 cm × 0.5 cm) (right). Figure S2: The X-ray diffraction pattern of the electrodeposited materials. Figure S3: Energy-dispersive X-ray (EDX) mapping images of electrodeposited materials. Figure S4: SEM images of the Ag7O8NO3 micro-pyramids’ morphology evolution under the different electrodeposition voltage. Figure S5: SEM images of the Ag7O8NO3 micro-pyramids’ morphology evolution under different electrodeposition time. Figure S6: Energy-dispersive X-ray (EDX) mapping images of nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. Figure S7: SERS signals of R6G (10−6 M) collected from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids reduced by using NaBH4 solution with different concentrations. Figure S8: Morphology of nanoporous Ag-decorated Ag7O8NO3 micro-pyramids reduced using 5 mM NaBH4 solution. Figure S9: SERS signals of R6G (10−7 M) collected from Ag7O8NO3 micro-pyramids and Ag7O8NO3 micro-pyramids by chemical reduction. Figure S10: SERS spectrum of 10−11 M R6G absorbed on the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids, and the normal Raman spectrum of R6G polyvinyl chloride film. Figure S11: Raman spectra of CV (10−5 M) and thiram (10−5 M), and the RSD values of 1618 cm−1 (CV) and 1384 cm−1 (thiram) peaks, respectively. Table S1: Comparison of LODs between various SERS substrates

Author Contributions

Conceptualization, L.G. and C.Z.; methodology, L.G. and C.Z.; validation, L.G. and C.Z.; formal analysis, L.G. and C.Z.; investigation, L.G.; resources, C.Z., Y.Y. and H.T.; data curation, L.G; writing—original draft preparation, L.G; writing—review and editing, C.Z., X.W. and H.T.; supervision, C.Z.; project administration, C.Z.; funding acquisition, C.Z. and H.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No 22175002 and 51972308) and Natural Science Foundation of Anhui Province (No 1808085J12, 2008085ME171 and 201903a07020021) for the experiments and characterizations.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Haichao Zhai of the University of Chemistry and Chemical Engineering, Anhui University, Hefei, China, for much helpful discussion of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemosensors 10 00539 sch001 550
Scheme 1. Schematic diagram of preparation process for the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (a) Ag7O8NO3 micro-pyramids are fabricated using electrodeposition in a pure aqueous AgNO3 solution with a two-electrode system. (b,c) Ag7O8NO3 micro-pyramids are transformed into nanoporous Ag-decorated Ag7O8NO3 micro-pyramids by NaHB4 reduction. (d) Nanoporous Ag-decorated Ag7O8NO3 micro-pyramids are used as SERS substrates to detect organic molecules.
Scheme 1. Schematic diagram of preparation process for the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (a) Ag7O8NO3 micro-pyramids are fabricated using electrodeposition in a pure aqueous AgNO3 solution with a two-electrode system. (b,c) Ag7O8NO3 micro-pyramids are transformed into nanoporous Ag-decorated Ag7O8NO3 micro-pyramids by NaHB4 reduction. (d) Nanoporous Ag-decorated Ag7O8NO3 micro-pyramids are used as SERS substrates to detect organic molecules.
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Figure 1. (a) SEM image of the Ag7O8NO3 micro-pyramids fabricated by electrodeposition at 3 V for 300 s. The inset is a magnified image. (b) SEM image of the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids prepared by reducing the surface of Ag7O8NO3 in a NaBH4 solution. The inset is a magnified image. (c) TEM image of nanoporous Ag, and the inset is the corresponding high-resolution transmission electron microscopy (HRTEM) image marked in the yellow box in (c). (d) TEM image of Ag nanostructure peeled off from a nanoporous Ag-decorated Ag7O8NO3 micro-pyramid through ultrasonic. (e) Corresponding elemental mapping of Ag collected form the area shown in (d). (f) XRD patterns of (upper curve) the prepared nanoporous Ag-decorated Ag7O8NO3 micro-pyramids and (lower curve) an ITO glass.
Figure 1. (a) SEM image of the Ag7O8NO3 micro-pyramids fabricated by electrodeposition at 3 V for 300 s. The inset is a magnified image. (b) SEM image of the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids prepared by reducing the surface of Ag7O8NO3 in a NaBH4 solution. The inset is a magnified image. (c) TEM image of nanoporous Ag, and the inset is the corresponding high-resolution transmission electron microscopy (HRTEM) image marked in the yellow box in (c). (d) TEM image of Ag nanostructure peeled off from a nanoporous Ag-decorated Ag7O8NO3 micro-pyramid through ultrasonic. (e) Corresponding elemental mapping of Ag collected form the area shown in (d). (f) XRD patterns of (upper curve) the prepared nanoporous Ag-decorated Ag7O8NO3 micro-pyramids and (lower curve) an ITO glass.
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Figure 2. SEM images of the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids fabricated by using NaBH4 solution with different concentrations: (a) 1 mM; (b) 5 mM; (c) 10 mM; and (d) 25 mM.
Figure 2. SEM images of the nanoporous Ag-decorated Ag7O8NO3 micro-pyramids fabricated by using NaBH4 solution with different concentrations: (a) 1 mM; (b) 5 mM; (c) 10 mM; and (d) 25 mM.
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Figure 3. (a) SERS spectra of 10−10, 10−12 and 10−14 M R6G collected from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (b) Linear relationship between the logarithmic intensities of 1648 cm−1 peak and the concentrations of R6G. (c) SERS spectra of R6G (10−6 M) at 10 randomly selected sites on the SERS substrate made of nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (d) The 1648 cm−1 band intensities of R6G (10−6 M) recorded from 10 randomly selected sites.
Figure 3. (a) SERS spectra of 10−10, 10−12 and 10−14 M R6G collected from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (b) Linear relationship between the logarithmic intensities of 1648 cm−1 peak and the concentrations of R6G. (c) SERS spectra of R6G (10−6 M) at 10 randomly selected sites on the SERS substrate made of nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (d) The 1648 cm−1 band intensities of R6G (10−6 M) recorded from 10 randomly selected sites.
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Figure 4. (a) SERS spectra of (Curve I) 10−8, (Curve II) 10−10 and (Curve III) 10−12 M CV collected from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (b) Linear relationship between the logarithmic intensities (1618 cm−1 peak) and the concentrations of CV. (c) SERS spectra of (Curve I) 10−7, (Curve II) 10−9 and (Curve III) 10−11 M thiram from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (d) Linear relationship between the logarithmic intensities (1384 cm−1 peak) and the concentrations of thiram.
Figure 4. (a) SERS spectra of (Curve I) 10−8, (Curve II) 10−10 and (Curve III) 10−12 M CV collected from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (b) Linear relationship between the logarithmic intensities (1618 cm−1 peak) and the concentrations of CV. (c) SERS spectra of (Curve I) 10−7, (Curve II) 10−9 and (Curve III) 10−11 M thiram from nanoporous Ag-decorated Ag7O8NO3 micro-pyramids. (d) Linear relationship between the logarithmic intensities (1384 cm−1 peak) and the concentrations of thiram.
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Guo, L.; Tang, H.; Wang, X.; Yuan, Y.; Zhu, C. Nanoporous Ag-Decorated Ag7O8NO3 Micro-Pyramids for Sensitive Surface-Enhanced Raman Scattering Detection. Chemosensors 2022, 10, 539. https://doi.org/10.3390/chemosensors10120539

AMA Style

Guo L, Tang H, Wang X, Yuan Y, Zhu C. Nanoporous Ag-Decorated Ag7O8NO3 Micro-Pyramids for Sensitive Surface-Enhanced Raman Scattering Detection. Chemosensors. 2022; 10(12):539. https://doi.org/10.3390/chemosensors10120539

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Guo, Linfan, Haibin Tang, Xiujuan Wang, Yupeng Yuan, and Chuhong Zhu. 2022. "Nanoporous Ag-Decorated Ag7O8NO3 Micro-Pyramids for Sensitive Surface-Enhanced Raman Scattering Detection" Chemosensors 10, no. 12: 539. https://doi.org/10.3390/chemosensors10120539

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