Simultaneous SERS detection using hexagonal hollow Au-Ag nanoparticles with near infrared plasmon
Introduction
Metallic nanostructures have received considerable attention in the past few decades owing to their exceptional optical and physico-chemical properties. Since, silver (Ag) and gold (Au) nanoparticles are predominant in offering many advantages compared to other metallic nanoparticles; they have been extensively employed in optical, electronic and catalytic processes [[1], [2], [3], [4], [5]]. Moreover, the strong localized surface plasmon resonances (LSPR) present in these metallic nanoparticles are responsible for their unique optical properties [[6], [7], [8]]. More interestingly, these optical properties can be fine-tuned according to the demand of the application by controlling size, shape and morphology of the nanoparticles. This indeed evokes the corresponding surface plasmon due to the interaction of nanoparticles with the electromagnetic field which induces a resonant oscillation of free electrons [[9], [10], [11]]. Therefore, Au and Ag nanoparticles can be used as a platform for surface-enhanced Raman spectroscopy (SERS), which enrich the Raman signal of analyte molecule by several orders of magnitude by intensifying the electron density around roughened metallic nanostructures. SERS is actually a non-invasive analytical technique providing selective, sensitive and non-destructive imprints of bio/chemical reactions, which is extremely required in numerous fields such as food safety, medical diagnostics, homeland security and environmental protection etc. [[12], [13], [14], [15]]. However, as compared to the Au metallic nanostructures, Ag acts as a better SERS substrate since it can generate plasmonic resonances over a wide range of electromagnetic wavelengths with relatively higher quality factors due to the interband transitions which take place at much higher frequencies than its LSPR [16]. But lack of stability and biocompatibility as compared to gold nanostructures backlashes it for the synthesis of reproducible SERS substrates that are biocompatible and give excellent signal to noise ratio, for potential biomedical application [17,18]. Thus, a hybrid nanostructure incorporating both silver and gold could combine the quality features of both the nanoparticles, especially in the near-infrared region [19]. Now a days, hollow nanostructures are used in many applications such as sensing, catalysis, and drug delivery because their cavities alter the plasmonic properties compared to their solid counterparts [20]. Consequently, hollow nanostructures of Ag and Au will be an exceptional SERS substrate by combining the properties of both Ag and Au nanoparticles. Moreover, by carefully adjusting the morphology and structural parameters, the optical properties of these Au-Ag nanostructures can be finely tuned and can be used as a promising candidate for various catalytic and biomedical applications [21,22].
The utilization of principal component analysis (PCA) has been shown to be effective in the analysis of SERS spectra of different analytes [[23], [24], [25]]. PCA is a multivariate analytic tool for identifying patterns in a data [26]. In this analysis, a high dimensional data set is projected on to a lower dimension, by eliminating components which contribute less to the information present in the data set. The compressed lower dimension data consists of uncorrelated variables known as principal components and the highest amount of variance is retained by the first principal component. The maximum variation, which is not modeled by the preceding principal components, is described by each succeeding principal component [27,28]. The transformed lower dimensions are represented as score plots and the association between the components and input variables are represented as loading plots. PCA finds application in many areas where the dimensionality of input data is huge [29,30].
In this work, we report the synthesis of hexagonal hollow Au-Ag nanostructures using galvanic replacement mechanism. Initially, hexagonal silver nanoparticles are obtained using a seed-mediated synthesis procedure, which act as sacrificial templates. These templates are used for fabricating hollow Au-Ag nanostructures. The dimensions of the cavity were varied by controlling the addition of oxidizing material chloroauric acid to the solid templates. The SERS activity of these hollow nanostructures was investigated using three probe molecules viz., crystal violet (CV), malachite green (MG) and nile blue chloride (NBC) and their mixture under two laser excitation sources viz., visible (514.4 nm) and near-infrared (784.8 nm). PCA was done on the enhanced Raman spectra of these molecules and their mix, so that these dyes and their mixture could be simultaneously detected. The corresponding score and loading plots were obtained in the transformed lower dimension. The score plots evidenced the segregation of probe molecules and the loading plots provided the relationship of the spectral data with the principal components.
Section snippets
Materials and synthesis methods
Silver nitrate (AgNO3, 99.95 %), polyvinylpyrrolidone (PVP, 99 %, Molecular weight- 40,000), sodium borohydride (NaBH4, 99 %), L-ascorbic acid (99 %) and gold(III) chloride trihydrate (HAuCl4, 99 %) were purchased from Sigma-Aldrich. Double-distilled water was used for the experimental methods. All glasswares were cleaned with aqua regia, followed by double distilled water, and dried before usage.
A solution mixture containing 0.25 mM AgNO3 and 5 mM trisodium citrate was stirred continuously for
Results and discussion
Silver nanostructures as templates with hexagonal shapes, were synthesized by a seed- mediated approach. At first, spherical shaped silver nanoseeds were produced by reducing silver nitrate with NaBH4. The seeds were stabilized using trisodium citrate. In order to promote the anisotropic growth of hexagonal shaped silver nano templates, the seed particles were added to a growth medium consisting of PVP as structure-directing agent, silver nitrate as the metal precursor and ascorbic acid as
Conclusions
Two interesting topics in nanotechnology viz., synthesis of hollow Au-Ag nanostructures and the simultaneous detection of different probe molecules from enhanced Raman signals employing them as substrates are presented. Homogeneous silver hexagonal nanoparticles synthesized using seed mediated strategy was used as sacrificial templates. The galvanic replacement mechanism was used for making Au-Ag hollow nanostructures. It was found that the size and depth of the cavities depend on the amount of
CRediT authorship contribution statement
C.R. Rekha: Conceptualization, Methodology, Data curation, Writing - original draft. S.G. Jiji: Conceptualization, Methodology, Data curation, Writing - original draft. V.U. Nayar: Supervision. K.G. Gopchandran: Supervision.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgement
Rekha C.R. and Jiji S.G. acknowledge DST-PURSE Programme for financial support.
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2022, Vibrational SpectroscopyCitation Excerpt :SERS is mainly attributed to the local electromagnetic (EM) enhancement caused by surface plasmon resonance of noble metal nanoparticles or chemical (CM) enhancement [9–11]. Noble metal nanoparticles (Au and Ag colloids) are widely used as SERS substrates based on their significant Raman-enhancing effect induced by localized surface plasmon resonance (LSPR) [12,13]. Furthermore, to achieve better SERS activity, nanoparticles with elaborate structures, such as nanoflowers [14], thorny nanoparticles [15], raspberry-like nanostructures [16], and sea urchin-like nanoparticles [17] have been prepared.