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Great influences of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the analyte and their alignments compared to the Fermi level of the substrate on the charge transfer (CT) process, and consequently, on the surfaceenhanced Raman scattering (SERS) phenomenon have been described via theoretical calculations. To provide experimental evidence, in this study, two antibiotics, chloramphenicol (CAP) and amoxicillin (AMX), were investigated as analytes in SERS sensors based on electrochemically synthesized colloidal silver nanoparticles (eAgNPs) as the substrate. Despite the same experimental condition, similarities in analyte structure, and in the ability of absorbing onto eAgNPs, the detection of the two antibiotics showed obvious distinction. While CAP was able to be detected using eAgNPbased SERS sensors at concentrations down to 1.2 × 10−9 M, there were no characteristic peaks observed in the SERS spectra of AMX even at a high concentration of 10−3 M. The LUMO and HOMO energy levels of the two analytes were measured using electrochemical cyclic voltammetry. The obtained results showed that the LUMO levels of both analytes were higher than the Fermi level of Ag, and the LUMO level of AMX was higher than that of CAP. The larger gap between the LUMO level of AMX and the Fermi level of Ag might have prevented the metaltomolecule CT process, which is related to the Raman signal enhancement in both chemical and electromagnetic mechanisms. In contrast, the smaller energy gap in the case of CAP might have allowed the transfer of hot electrons from the Fermi level of the eAgNPs to the LUMO level of the analyte. Therefore, CAP could experience an SERS effect on the eAgNPs under the excitation of a 785 nm laser source, while AMX could not. The hypothesis was then confirmed using three other organic compounds, including furazolidone, 4nitrophenol, and tricyclazole. The results revealed a clear correlation between the LUMO level of the analytes and their SERS signals.
pubs.acs.org/JPCC Article Silver Nanoparticles-Based SERS Platform towards Detecting Chloramphenicol and Amoxicillin: An Experimental Insight into the Role of HOMO−LUMO Energy Levels of the Analyte in the SERS Signal and Charge Transfer Process Downloaded via Mai Doan on April 27, 2022 at 05:10:12 (UTC) See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles Quan Doan Mai,# Ha Anh Nguyen,*,# Thi Lan Huong Phung, Ngo Xuan Dinh, Quang Huy Tran, Tri Quang Doan, and Anh-Tuan Le* Cite This: https://doi.org/10.1021/acs.jpcc.2c01818 ACCESS Metrics & More Read Online Article Recommendations sı Supporting Information * ABSTRACT: Great influences of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the analyte and their alignments compared to the Fermi level of the substrate on the charge transfer (CT) process, and consequently, on the surface-enhanced Raman scattering (SERS) phenomenon have been described via theoretical calculations To provide experimental evidence, in this study, two antibiotics, chloramphenicol (CAP) and amoxicillin (AMX), were investigated as analytes in SERS sensors based on electrochemically synthesized colloidal silver nanoparticles (e-AgNPs) as the substrate Despite the same experimental condition, similarities in analyte structure, and in the ability of absorbing onto e-AgNPs, the detection of the two antibiotics showed obvious distinction While CAP was able to be detected using e-AgNP-based SERS sensors at concentrations down to 1.2 × 10−9 M, there were no characteristic peaks observed in the SERS spectra of AMX even at a high concentration of 10−3 M The LUMO and HOMO energy levels of the two analytes were measured using electrochemical cyclic voltammetry The obtained results showed that the LUMO levels of both analytes were higher than the Fermi level of Ag, and the LUMO level of AMX was higher than that of CAP The larger gap between the LUMO level of AMX and the Fermi level of Ag might have prevented the metal-to-molecule CT process, which is related to the Raman signal enhancement in both chemical and electromagnetic mechanisms In contrast, the smaller energy gap in the case of CAP might have allowed the transfer of hot electrons from the Fermi level of the e-AgNPs to the LUMO level of the analyte Therefore, CAP could experience an SERS effect on the eAgNPs under the excitation of a 785 nm laser source, while AMX could not The hypothesis was then confirmed using three other organic compounds, including furazolidone, 4-nitrophenol, and tricyclazole The results revealed a clear correlation between the LUMO level of the analytes and their SERS signals INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) is a Raman technique based on plasmonic materials, which has been developed for ultrasensitive detection of various analytes at low concentrations, and even at single-molecule level.1−3 Under the excitation of light, collective oscillations of the conductive electrons of the plasmonic materials generate a strong electromagnetic field on the material surface, which couples with the vibrational modes of the analyte adsorbed on the surface and leads to enhancement of its characteristic Raman signal.1,4 A strong electromagnetic field and formation of chemical complexation of the analyte to the metallic surface are both essential for the SERS phenomenon They are also the main principles of the two fundamental mechanisms explaining this giant enhancement of Raman signal: electromagnetic and chemical mechanisms (EM and CM), respectively.1,4 There© XXXX American Chemical Society fore, to optimize the SERS performance, many strategies have been used targeting these two mechanisms, focusing on material fabrication and surface modification For example, on one hand, EM enhancement increases in several specific regions, called hotspots, including the sharp tips of nanoparticles (NPs) and the nanogaps ( EHOMO (a), EHOMO > Φm > ELUMO (b), Φm < ELUMO (c), where Φm is the work function of the metal surface where λ is the wavelength, c is the speed of light in vacuum, and h is the Planck constant Our calculations from the electrochemical measurements showed the HOMO−LUMO G https://doi.org/10.1021/acs.jpcc.2c01818 J Phys Chem C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure Proposed hot-electron transfer process from the e-AgNPs to the analyte in two cases: hot electrons from the surface of the e-AgNPs can transfer to CAP, with a low energy difference between EF and ELUMO (ELUMO−CAP − EF = 0.42 eV) (panel 1), and hot electrons from the surface of e-AgNPs cannot transfer to the AMOX, with a large energy difference between EF and ELUMO (ELUMO−AMOX − EF = 0.89 eV) (panel 2) Figure Determination of the HOMO−LUMO energy levels of FZD, 4-NP, and TCZ analytes via cyclic voltammetry measurements to a transition between the Fermi level of the metal and the LUMO level of the analyte The difference between the Fermi energy level of e-AgNPs and the LUMO energy level of CAP was calculated to be 0.42 eV, which was obviously lower than that of AMX (i.e., 0,89 eV) This significant distinction may be the explanation for the difference in the SERS results of the two analytes described in Section 3.1 The gap of 0.89 eV might have been so large that it was impossible for a CT transition from the Fermi level of the e-AgNPs to the LUMO level of AMX On the other hand, a smaller gap of 0.42 eV allowed the occurrence of this transition in the case of CAP We propose hot-electron transfer processes from the e-AgNPs to CAP and AMX as shown in Figure 8, exhibiting the absorbed molecule: (a) the work function of the metal surface (Φm) is larger than the HOMO energy level of the analyte (Figure 7a); (b) Φm is less than the HOMO energy level of the analyte and larger than the LUMO energy level of the analyte (Figure 7b); and (c) Φm is less than the LUMO energy level of the analyte (Figure 7c) Otero et al proposed that the electron transfer from the metal to analyte or the analyte to metal would occur even without any laser excitation in two cases: Φm < ELUMO and Φm > EHOMO.21 The Fermi energy level of Ag was reported to be approximately −4.26 eV.31 Thus, the Fermi energy level of e-AgNPs and HOMO−LUMO levels of CAP and AMX is in the case (b) Consequently, to achieve SERS effect, e-AgNPs have to be excited by laser irradiation, leading H https://doi.org/10.1021/acs.jpcc.2c01818 J Phys Chem C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 10 SERS spectra of FZD (10−3−10−5 M) obtained by preparing three different e-AgNP/Al substrates Figure 11 (a) SERS spectra of TCZ (10−3−10−10 M) obtained on e-AgNP/Al substrates (b) Plot of log of SERS intensity−concentrations at 1360 cm−1 (slope 0.337 ± 0.02, intercept 4.53 ± 0.01) (c) SERS spectra of TCZ (10−6−10−10 M) could be made, and comparison between the predicted and actual signals might be a confirmation for the correlation between the HOMO−LUMO energy levels of the analytes and their SERS signals To be convenient for SERS detection, the three selected compounds, including furazolidone (FZD), 4nitrophenol (4-NP), and tricyclazole (TCZ), were all reported to be able to adsorb onto the e-AgNP surface.32−34 CV measurements of those organic compounds were carried out in 0.1 M PBS (Figure 9) In the absence of analyte, no redox peak was detected In the presence of FZD, 4-NP, and TCZ, irreversible anodic peaks appear at 1.28, 0.88, and 1.76 V, respectively They are the onset oxidation peaks of FZD, 4NP, and TCZ, respectively The onset oxidation potential (ϕox) values of FZD, 4-NP, and TCZ were measured to be 1.1, 0.7, and 1.14 V, respectively Using eq (2), the HOMO levels importance of the gap between the LUMO level of the analyte and the Fermi energy level of the substrate to the SERS signal However, our experimental data did not show any significant evidence for the effects of HOMO level of the analyte on its SERS signal 3.4 Confirmation for the Effects of HOMO−LUMO Energy Levels of the Analyte on the SERS Signal: Examples of Several Other Analytes The experimental model of CAP and AMX has shown the importance of the LUMO energy level of the analyte on SERS signal However, HOMO level did not show any significant effects To better confirm this hypothesis, we carried out some “quick tests” on several organic compounds, in which their HOMO−LUMO levels were, at first, estimated via CV measurements From these energy levels, several predictions of their SERS signals I https://doi.org/10.1021/acs.jpcc.2c01818 J Phys Chem C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 12 SERS spectra of 4-NP (10−3−10−5 M) were obtained by preparing three different e-AgNP/Al substrates plot of the logarithmic SERS intensity at 1360 cm−1 against TCZ concentration, based on which LOD was calculated to be 5.86 × 10−10 M, which is significantly lower than that when detecting CAP Therefore, the smaller gap between the LUMO level of the analyte and the Fermi level of the substrate resulted in a higher intensity of the SERS signal, leading to better sensitivity of the SERS sensor As a more complicated case, the LUMO level of 4-NP is higher than that of CAP but lower than that of AMX Thus, there could be two possibilities for the SERS signal of 4-NP (1) In case the gap between the LUMO level of the analyte and the Fermi level of Ag was still too large for the CT process to occur, no SERS signal would be obtained (2) In case the gap was small enough for the CT process, the SERS signal of 4NP on e-AgNPs could be recorded, but with lower intensity compared to that of CAP The SERS results were in agreement with the second possibility, as at high concentrations of 4-NP (10−3 and 10−4 M), SERS spectra show characteristic bands at 858, 1270, 1330, 1486, and 1580 cm−1 (Figure 12) The band at 858 cm−1 represents the bending mode of the nitro group.38,39 The band at 1330 cm−1 is assigned to the symmetric stretching of the nitro group, while the band at 1270 cm−1 is attributable to a ring deformation mixed with the stretching mode of the nitro group.38,39 The band at 1486 cm−1 represents the bending mode of C−H.39 The band at 1580 cm−1 is assigned to the stretching mode of the ring.38,39 However, the SERS intensity is much lower than that of CAP at the same concentration These peaks disappear at the concentration of 10−5 M Similar to the experiments on FZD, the SERS measurements of 4-NP were also repeated times, revealing the same results (Figure 12) Furthermore, it is worth mentioning that CAP is a nitrophenyl-substituted molecule The high intensity of the band at 1350 cm−1 in the SERS spectra of CAP and the bands at 1330 and 1270 cm−1 in the SERS spectra of 4-NP suggests that the adsorption of both CAP and 4-NP occurs via their nitro groups.19,39 Moreover, the smaller molecular weight and steric hindrance should have been the advantages of 4-NP to adsorb onto the e-AgNP surface to experience a better SERS effect However, the actual SERS result is the opposite It should be explained by the difference in LUMO levels of these two compounds of FZD, 4-NP and TCZ were calculated to be −5.46, −5.06, and −5.75 eV, respectively On the other hand, the onset reduction peaks of FZD, 4-NP, and TCZ were determined thanks to the appearance of irreversible cathodic peaks at −1.53, −0.94, and −0.65 V, respectively The onset reduction potential (ϕred) values of FZD, 4-NP, and TCZ were estimated to be −1.1, −0.85, and −0.4 V, respectively The LUMO levels of FZD, 4-NP, and TCZ were calculated to be −3.26, −3.55, and −3.98 eV, respectively The values were averaged over 10 cycles of CV scans (Figure S5) Obviously, the LUMO level of FZD is even higher than that of AMX; therefore, it was expected that FZD would not be detected using a SERS sensor based on e-AgNPs under 785 nm laser irradiation This prediction was then confirmed by the SERS measurements of FZD on e-AgNPs (Figure 10) Similar to the results obtained in the presence of AMX, no characteristic peak was observed in the SERS spectra of FZD, even at a concentration as high as 10−3 M The experiments were repeated times, obtaining similar results It is interesting that the SERS signal of FZD is similar to that of AMX, while FZD is 30% smaller than AMX in molecular weight, and possesses lower steric hindrance and a different functional group (−NO2) (Figure 13) This result stresses the effect of the LUMO level of analytes on their SERS signals In contrast, the LUMO level of TCZ is lower than that of CAP The smaller gap between the LUMO level of TCZ and the Fermi level of Ag was expected to be convenient for CT, and consequently, TCZ was predicted to be detectable using the e-AgNP-based SERS sensing system The actual result was in agreement with the prediction Figure 11a,c shows the SERS spectra of TCZ at different concentrations (10−10−10−3 M), with characteristic peaks at 430, 566, 592, 890, 1283, and 1360 cm−1 The band at 430 cm−1 is assigned to the deformation of the C−N−C vibration.35−37 The band at 566 cm−1 represents the deformation of C−C bending.35−37 The band at 592 cm−1 is associated with the C−S−C vibration mode.35−37 The band at 890 cm−1 represents the symmetric stretching mode of C C vibration.35−37 The bands at 1283 and 1360 cm−1 are assigned to the C−N stretching vibration.35−37 At the same concentration, the SERS spectra of TCZ exhibit a higher intensity in comparison to that of CAP Figure 11b shows the J https://doi.org/10.1021/acs.jpcc.2c01818 J Phys Chem C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 13 Effects of the LUMO energy level of some analytes on their SERS signals The “quick tests” on FZD, TCZ, and 4-NP confirmed the effects of LUMO level of the analytes on their SERS signals Figure 13 shows the correlation between the LUMO level of the analytes and the intensity of their SERS signals The increase of the LUMO level of the analyte enlarges the gap between the Fermi level of the substrate and the analyte, leading to the decrease of SERS intensity Once the gap is large enough to prevent the CT process, the SERS phenomenon cannot occur, resulting in the absence of SERS signal The HOMO level, however, did not show any clear effects on SERS signal in this study These results emphasized the importance of the LUMO level in the CT process, and consequently, the SERS phenomenon Moreover, it provides an explanation for the impossibility of detection of several analytes by SERS sensors A step of measuring the LUMO level of the analyte before designing and fabricating SERS sensors was also recommended This understanding suggests researchers improving their SERS sensors by minimizing the gap between the Fermi level of the substrate and the LUMO level of the desired analyte, which can orientate the fabrication of SERS materials in the future respectively, while the EHOMO and ELUMO of AMX were estimated to be −5.53 and −3.37 eV, respectively Both of the LUMO energy levels were higher than the Fermi level of Ag (−4.26 eV) Therefore, the antibiotics could experience SERS only during the plasmon-induced CT transition from the Fermi level of the e-AgNPs to the LUMO energy levels of the analytes A smaller gap of 0.42 eV from Fermi level of the eAgNPs to the LUMO level of CAP might have been more convenient for the process In contrast, the gap of 0.89 eV of AMX seemed to be too large for CT; therefore, AMX was unable to be detected by the e-AgNP-based SERS sensors under the excitation of a 758 nm laser source Thus, the obvious difference in the ability to be detected via SERS sensors of the two analytes was explained by the distinction in LUMO energy levels, which is important for Raman signal enhancement in both CM and EM, especially for the CT process However, this experimental model did not show any clear effects of HOMO level on the SERS signal The hypothesis about the effects of LUMO level of the analyte on the SERS signal was confirmed using three other organic compounds: furazolidone, 4-nitrophenol, and tricyclazole These results emphasize the importance of the LUMO level of the analyte in a metal-to-molecule CT process, and therefore, in the SERS effect It provides an explanation for the question why several analytes could not be detected by SERS sensors Besides, this understanding suggests a step of measuring the LUMO level of the desired analytes before designing SERS sensors The idea of minimizing the gap between the Fermi level of the substrate and the LUMO level of the analyte can orientate future trends of SERS material fabrication CONCLUSIONS e-AgNPs were employed to develop SERS sensors for two antibiotics, CAP and AMX In spite of the similarities in the structure of the two analytes, SERS experiments, which were carried out under the same condition, showed completely different results While CAP could be detected by SERS sensing at concentrations down to 1.2 × 10−9 M, no characteristic band was observed in the SERS spectra of AMX, even at a high concentration of 10−3 M Since the adsorption of the analyte onto the metal surface of the substrate is essential for the SERS phenomenon, the adsorption ability of CAP and AMX was examined However, the adsorption of two analytes was confirmed by UV−vis Their LUMO and HOMO levels were measured using electrochemical cyclic voltammetry The EHOMO and ELUMO of CAP were calculated to be −5.61 and −3.84 eV, ■ ASSOCIATED CONTENT sı Supporting Information * The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c01818 Calculation of LOD, LOQ, EF and RSD; Raman spectrum of CAP powder; molecular structure of CAP; K https://doi.org/10.1021/acs.jpcc.2c01818 J Phys Chem C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C ■ pubs.acs.org/JPCC (2) Á lvarez-Puebla, 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12116, Vietnam; orcid.org/0000-0002-1183-5041; Email: anh.nguyenha@phenikaa-uni.edu.vn Anh-Tuan Le − Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Vietnam; Faculty of Materials Science and Engineering, Phenikaa University, Hanoi 12116, Vietnam; Email: tuan.leanh@ phenikaa-uni.edu.vn Authors Quan Doan Mai − Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Vietnam; orcid.org/0000-0003-2931-0822 Thi Lan Huong Phung − Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Vietnam; Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi 10000, Vietnam Ngo Xuan Dinh − Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Vietnam Quang Huy Tran − Phenikaa University Nano Institute (PHENA), Phenikaa University, Hanoi 12116, Vietnam; Faculty of Electric and Electronics, Phenikaa University, Hanoi 12116, Vietnam Tri Quang Doan − Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), Hanoi 10000, Vietnam Complete contact information is available at: https://pubs.acs.org/10.1021/acs.jpcc.2c01818 Author Contributions # Q.D.M and H.A.N contributed equally to this work Q.D.M: Conceptualization, Validation, Investigation, WritingOriginal Draft; H.A.N.: Conceptualization, Methodology, Formal Analysis, WritingOriginal Draft; T.L.H.P.: Validation, Investigation; N.X.D.: Conceptualization, Formal Analysis; T.Q.D.: Methodology, Formal Analysis; Q.H.T.: Methodology, Validation; A.T.L: Conceptualization, Methodology, Supervision, Project administration, Writingreview & editing Notes The authors declare no competing financial interest ■ ACKNOWLEDGMENTS This research was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) through a fundamental research project (103.02−2019.01) The authors would like to acknowledge the support for Raman, electrochemical 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thiram residues in pear extracts using SERS Food Chem 2021, 350, No 129025 (38) Hostert, L.; Blanc, C.; Zarbin, A J G.; Anglaret, E.; Orth, E S SERS detection and comprehensive study of p-nitrophenol: towards pesticide sensing New J Chem 2021, 45, 3886−3891 Article (39) Muniz-Miranda, M SERS monitoring of the catalytic reduction of 4-nitrophenol on Ag-doped titania nanoparticles Appl Catal., B 2014, 146, 147−150 M https://doi.org/10.1021/acs.jpcc.2c01818 J Phys Chem C XXXX, XXX, XXX−XXX ... the effects of LUMO level of the analytes on their SERS signals Figure 13 shows the correlation between the LUMO level of the analytes and the intensity of their SERS signals The increase of the. .. of the analyte on its SERS signal 3.4 Confirmation for the Effects of HOMO−LUMO Energy Levels of the Analyte on the SERS Signal: Examples of Several Other Analytes The experimental model of CAP and. .. level of the analyte (Figure 7a); (b) Φm is less than the HOMO energy level of the analyte and larger than the LUMO energy level of the analyte (Figure 7b); and (c) Φm is less than the LUMO energy