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Cost-Effective Plasmonic Platforms: Glass Capillaries Decorated with Ag@SiO2 Nanoparticles on Inner Walls as SERS Substrates M. Shanthil,†,‡ Hemna Fathima,† and K. George Thomas*,† †
School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM), CET Campus, Thiruvananthapuram 695 016, India ‡ Photosciences and Photonics, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695 019, India S Supporting Information *
ABSTRACT: A cost-effective method for the fabrication of a glass capillary based plasmonic platform for the selective detection and identification of analytes of importance in health, environment, and safety is demonstrated. This was achieved by coating Ag@SiO2 nanoparticles (Ag ∼ 60 nm) having silica shell of varying thickness (∼2 and ∼25 nm) on the inside walls of glass capillaries, over 2 cm in length, with uniform coverage. It was found that the particle density on the surface plays a decisive role on the enhancement of Raman signals. Multiple hot spots, which are essentially junctions of amplified electric field, were generated when ∼30 Ag@SiO2 particles/μm2 were bound onto the walls of glass capillaries. The pores of the silica shell allow the localization of analyte molecules to the vicinity of hot spots resulting in signal enhancements of the order of 1010 (using pyrene as analyte; excitation wavelength, 632.8 nm). The applicability of Ag@SiO2 coated capillaries for the detection of a wide range of molecules has been explored, by taking representative examples of polyaromatic hydrocarbons (pyrene), amino acids (tryptophan), proteins (bovine serum albumin), and explosives (trinitrotoluene). By increasing the thickness of the silica shell of Ag@SiO2 nanoparticles, an effective filtration cum detection method has been developed for the selective identification of small molecules such as amino acids, without the interference of large proteins. KEYWORDS: plasmonic platform, SERS, Ag nanoparticle, silica shell, hot spot, capillary tube
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the low Raman scattering cross-section of molecules (∼10−25 to 10−31 cm2 per molecule) limit their applications.11,14 One can conveniently enhance the Raman spectroscopic signals of molecules by bringing them to the vicinity of the plasmonic field.15 It was recently demonstrated that plasmonic effects are significantly high when the gap between nanoparticles is less than 15 nm and the Raman signal enhancement factors at the hot spots follow a distance (d) dependence of 1/dn with n = 1.5, which is in good agreement with the theoretical reports.13,16 These studies provided fundamental insight on plasmon coupling and surface-enhanced spectroscopy. Arrays of well-organized plasmonic structures with precise nanogaps can generate multiple hot spots and are proposed as substrates for surface-enhanced Raman spectroscopy (SERS) for the detection and quantification of various analyte molecules.17−20 In such an arrangement, when analyte molecules are trapped anywhere on the plasmonic platform, it experiences high
INTRODUCTION The surface plasmons in metal nanoparticles, especially Ag, Au, and Cu, can be directly excited by freely propagating electromagnetic radiation in the visible region, which in turn generates an intense electric field around the nanostructures.1−4 The presence of this intense electric field enhances the spectroscopic signals of the molecule by several orders of magnitude.5−7 When two or more plasmonic nanostructures are placed in close proximity, in the order of a few nanometers, substantial enhancement in the spectroscopic signal occurs. The oscillating electric fields of plasmonic nanostructures, separated by well-defined nanogaps, interact to yield new resonances having an amplified electric field at their junction termed as hot spots.1,8,9 Due to the presence of this amplified electric field, spectroscopic signals (e.g., Raman signals) of molecules trapped at the hot spot show huge enhancement as compared to the molecules bound on the surface of an isolated particle.10−13 When various spectroscopic techniques are considered, Raman spectroscopy is proposed as a versatile tool for the rapid identification and quantification of a variety of biologically and environmentally important molecules. This is essentially due to the fact that Raman lines are the spectral fingerprint of a molecule. However, weak signals arising due to © XXXX American Chemical Society
Special Issue: Focus on India Received: September 30, 2016 Accepted: February 14, 2017
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DOI: 10.1021/acsami.6b12478 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Various steps involved in the fabrication of glass capillary tubes coated with Ag@SiO2 nanoparticles in their inner wall: (A) TEM images of PVP stabilized Ag nanoparticles before and after overcoating with SiO2 using tetraethyl orthosilicate (TEOS). (B) TEM images of Ag@SiO2 NPs having SiO2 shell of ∼2 and ∼25 nm, obtained by controlling the reaction time. TEM images were recorded by drop-casting the solution onto a carbon coated Cu grid. (C) Functionalization of Ag@SiO2 NPs onto the inner walls of a glass capillary tube involving aspiration and drying steps. Right side shows the photograph of Ag@SiO2 NPs (t ∼ 2 nm) coated capillary tube and schematic illustration of Ag@SiO2 NPs functionalized onto the inner walls of glass capillary tube.
these films act as molecular sieves, restricting the diffusion of large molecules (proteins).41 Principles of host−guest chemistry were also adopted to bring analyte molecules of interest to the vicinity of plasmonic nanostructures. Since a host molecule is specific to a particular analyte molecule, it is difficult to design a universal platform based on this methodology.24,42 This report demonstrate the direct functionalization of Ag@ SiO2 nanoparticles (Ag@SiO2 NPs), having varying thickness of SiO2 shell, onto the inside walls of a glass capillary through Si−O−Si bond formation. The use of Ag@SiO2 nanoparticles has several obvious advantages compared to the direct linkage of bare metal nanoparticles. The silica shell (i) prevents the coagulation of metal nanoparticles in the presence of analyte molecules, (ii) is optically transparent and chemically inert,43 and (iii) can be easily functionalized on glass capillaries.44 Also, the porosity of the silica shell can be tuned from micro- to macroscale allowing selective diffusion of molecules. Depending on the precursor, the charge as well as other surface characteristics (hydrophobic versus hydrophilic) of the silica shell can be tuned. Most importantly, silica shell spacer can vary the distance between the plasmonic nanoparticles and also can act as a template for holding the molecules of interest at the hot spots. Herein, we report the use of Ag@SiO2 coated glass capillaries as plasmonic platforms for the detection and quantification of various molecular systems ranging from polyaromatics to proteins to explosives.
electric field extending the detection limit of SERS to even at a single molecular level.6,21−23 However, the design of wellorganized, large area plasmonic platforms with high electric field and cost effectiveness is a major challenge. Lithographic methods have been utilized for the design of periodic arrays of plasmonic nanostructures having different size and shape, separated by well-defined nanogaps over a wide range, with uniformity and reproducibility.24,25 However, the difficulty of creating gaps below 10 nm limits the use of lithographic techniques.7,26−28 Recently, there have been numerous efforts to design cost-effective plasmonic substrates by adopting simple chemical methods.29 Coating of metal nanostructures has been achieved on solid substrates such as glass capillaries, glass slides, silicon wafers, and ITO plates, either by in situ reduction of metal salts or chemical vapor deposition methods.9,21,30−36 It may be noted that the former method has no control over the size and organization of nanoparticles whereas the latter method involves multistep procedures. Chemical functionalization of metal nanoparticles on various substrates is yet another method adopted by various groups for the design of SERS substrates. Attempts have been made to directly functionalize metal nanoparticles onto the glass substrate (planar sheet as well as capillary tubes) using linker groups such as (3-aminopropyl)trimethoxysilane.21,37−40 Liz-Marzan and co-workers have demonstrated the use of composite films comprising branched gold nanoparticles embedded in mesoporous thin films for SERS detection. While allowing the diffusion of small molecules of interest, B
DOI: 10.1021/acsami.6b12478 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Forum Article
ACS Applied Materials & Interfaces
Figure 2. Optical and electron microscopic characterization of Ag@SiO2NPs (t ∼ 2 nm) coated glass capillary tube achieved after eight cycles, keeping the concentration of Ag@SiO2 NPs as 0.4 nM: (A) Optical microscopic image recorded under transmission mode. (B) SEM images illustrating more or less uniform coverage (note: capillary tube was broken for imaging). (C) Magnified image of the Ag@SiO2 NPs coated surface and (D) SEM image of a broken section of the capillary tube.
Figure 3. SEM images of glass capillaries having varying particle density of Ag@SiO2 NPs (A−C) and the corresponding SERS spectrum (D−F) of pyrene (10−9 M). Particle density: (A) ∼10 and (B) ∼30 particles /μm2 (both prepared by following method A); (C) ∼80 particles /μm2 (prepared by following method C).
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molecules experience enhanced electric field and SERS. In contrast, molecules bound on Ag@SiO2 with higher t show negligible enhancement. Silver nanoparticles, prepared by sodium citrate reduction method were first coated with poly(vinylpyrrolidone) to make it vitreophilic. By following the Stöber condensation reaction, overcoating of Ag nanoparticles with silica was achieved and shell thickness was varied by controlling the reaction time. TEM images of Ag@SiO2 NPs are presented in Figure 1. The thin silica coating of ∼2 nm on silver nanoparticles is evident from the HR-TEM images presented as Supporting Information. Further, these Ag@SiO2 NPs were functionalized onto the inner walls of glass capillaries through Si−O−Si bond formation. Details are provided in the Experimental Section. Attempts were made to control the distribution and density of nanoparticles on the glass surface by increasing the coating cycles and the concentration of Ag@SiO2 NPs (designated as methods A and B, respectively; vide infra). In both of these methods, one of the open edges of the capillary tube was brought in such a way that it just touches the Ag@SiO2 NP solution of the desired particle concentration. The nanoparticles get aspirated into the tube due to the capillary force. The tubes were kept vertical in such a way that both edges are
RESULTS AND DISCUSSION Recently we have designed dimeric nanostructures of Ag@SiO2 by systematically varying the SiO2 shell thickness in the range of 1.5−40 nm and demonstrated the tunability of coupled electric field between the two Ag nanoparticles.13 When the distance between the Ag nanoparticles is kept below 5 nm, an intense electric field was observed at their junction (termed as hot spots). The molecules localized at the hotspots show large signal enhancement.13 The porosity of the silica shell is an additional advantage allowing the diffusion of small molecules into the silica cavity, filtering out larger molecules, which often interfere with the SERS measurements.41 The conclusions drawn from these reports form the basis of the design of the plasmonic platform in the present investigation. Silver nanoparticles having an average diameter of 60 ± 5 nm were synthesized and overcoated with SiO2, keeping the shell thickness (t) as ∼2 and ∼25 nm (Figure 1A,B). Earlier we have used the FDTD method to calculate the electric field experienced by molecules on Ag@SiO2 NPs by varying t.13 For example, the molecules on the silica surface will experience 88 and 8% of the total field on the Ag surface for t ∼3 and ∼25 nm, respectively. Based on theoretical and experimental investigations, we have demonstrated that when t ≤ 10 nm, C
DOI: 10.1021/acsami.6b12478 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Forum Article
ACS Applied Materials & Interfaces
Figure 4. (A) SERS spectra of pyrene recorded at different locations of Ag@SiO2 coated capillary tube and photograph of Ag@SiO2 coated capillary tube (left). (B) Time-dependent enhancement of Raman signal of pyrene monitored at a peak of 1240 cm−1 in Ag@SiO2 (t ∼ 2 nm) coated capillary tube. (C) Schematic illustration of diffusion of pyrene molecules through the pores of silica shell toward the silver core.
the incubation vessel. Ag@SiO2 NP solution was allowed to stand inside the capillary tubes for 1 day. This methodology provided a uniform and complete coating of nanoparticle in the inside walls of the capillary tube at a height of 0.5−1.0 cm from the bottom of the tube. From the SEM images, it was found that the Ag@SiO2 NPs on the inside wall of the glass capillary, prepared by adopting method C, were thickly packed (Figure 3C). However, achieving uniform coating beyond 1 cm from the bottom was found to be difficult. The particle density was calculated from the SEM images as ∼80 particles/μm2 Efficiency of capillary tubes prepared by two different drying procedures (methods A and C) in enhancing the Raman signal was further investigated (Figure 3). The analyte solution of a given concentration was aspirated into the capillary tube coated with Ag@SiO2 as shown in Scheme S1, keeping the time of contact as minimum as possible (
