Gravure printed paper based substrate for detection of heavy metals using surface enhanced Raman spectroscopy (SERS) Ali Eshkeiti, Morteza Rezaei, Binu. Baby Narakathu, Avuthu Sai Guruva Reddy, Sepehr Emamian, Massood Zandi Atashbar Department of Electrical and Computer Engineering, Western Michigan University Kalamazoo, MI, USA
[email protected]. Abstract— A novel paper based surface enhancemed Raman spectroscopy (SERS) substrate was fabricated by gravure printing single and double layers of silver nanoparticle (NP) ink, with a particle size of ~20-50 nm, as metallization layer on a paper from Mitsubishi (NB-RC3GR120). The capability of the SERS substrate for detection of toxic heavy metal compounds such as mercury sulfide (HgS) was demonstrated. The SERS based response of the printed substrate produced an enhanced Raman signal when compared to target molecules adsorbed on bare paper. An enhancement factor of five orders of magnitude, due to existence of hot spots between NP, was obtained. In addition, the effect of bending of the flexible paper substrate on the intensity of the Raman spectrum was also investigated. An enhancement of 500 % in the intensity of Raman spectra was obtained for a bending of 70°. The SERS based response of the printed substrate is analyzed and presented in this paper.
I.
INTRODUCTION
The toxic nature of heavy metals along with its harmful effects on both flora and fauna has gained increasing attention in the last few decades [1, 2]. The presence of heavy metal compounds, even at micro molar concentrations, has a harmful effect on human body, animal life and nature [3]. These common environmental pollutants such as mercury (Hg) and cadmium (Cd) which can be found in water and soil cause dangerous effects on the brain and nervous system as well as several types of bone diseases [4,]. Even though heavy metal compounds such as mercury sulfide (HgS) and cadmium sulfide (CdS) are not as dangerous as Hg and Cd, monitoring of these compounds as a reference point or step towards detection of Hg or Cd is very important. Different techniques such as UV-vis spectroscopy [5], plasma mass spectrometry [6], electrochemical impedance spectroscopy (EIS) [7] and colorimetric analysis [8] have been reported for the detection of heavy metals. The limitations associated with these methods include longer time consumption, requirement of specific tagging and expensive processes [9]. Because of these problems, there has been a need for the use of alternative methods to surpass the drawbacks of these traditional detection techniques.
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Surface enhanced Raman spectroscopy (SERS) has been used as a tool for detection of heavy metal compounds due to its ability to provide real time molecular level vibrational information, high degree of sensitivity, selectivity and acquisition of the Raman spectra in a very short period of time [10-12]. The most important advantage of SERS is its capability to provide unique structural information of the molecule under measurement. This ability enables SERS to detect and trace various heavy metals almost instantaneously. Compared with different methods, SERS can operate as a sensing method for detection of heavy metals without any need of sample treatment or labeling which makes it a fast and remarkable technique for the detection of any molecule [10]. Raman scattering can be described based on the theory of inelastic scattering of light. This scattering happens because of the interaction between a laser source and the vibrational modes or states of a molecule. As a result, the spectrum of the emitted molecule provides a structural fingerprint of that molecule which has interacted with the incident light. Studies have shown significant enhancement in the intensity of Raman spectra in the presence of metallic (NPs) such as silver (Ag) [13]. An enhancement in the Raman signal is achieved by the effect of Plasmon resonance which occurs due to the interaction between light and a metal surface [14, 15]. The ability of metals to reflect light very well along with its conductive properties is due to the existence of free electrons in the conduction layers [14]. To obtain the enhancement in the intensity of Raman spectra, the target molecules should be very close or in between the NPs. The optical frequency created by these NPs due to presence of free electrons in the conduction layer of metals creates an electromagnetic field which enhances the Raman spectra [14, 16]. Usually, the deposited NPs in molecular scale are not uniform; therefore different electromagnetic fields are obtained based on the position of the NPs. At some points very high electromagnetic field can be achieved and these points are called as hot spots. The NPs can be deposited on the substrates using different methods such as spin coating [17] lithography [18], and chemical vapor deposition [19]. The disadvantages of these
deposition techniques include turn-around time, expensive procedures and complex methods. These limitations could be overcome by employing the use of traditional printing techniques. Recent studies have shown the use of gravure printing as a method for the deposition of NP [20, 21]. In this work, we further characterized the paper based gravure printed SERS substrate for detection of mercury sulfide (HgS). In addition, to the effect of bending of the substrate on enhancement of Raman spectrum was investigated. II.
EXPRIMENTAL
A. Chemicals and Sample Preparation For fabrication of the SERS substrate, a paper sheet from Mitsubishi (NB-RC3GR120) was used as a substrate. Metallization was applied by gravure printing a Ag NP ink (TEC-PR-020) in aqueous form from Inktec. Heavy metal solutions (50 mM and 10 µM) were prepared by suspending crystalline HgS, purchased from Sigma-Aldrich Chemical Company, with deionized (DI) water. B. Sensor Fabrication Single and double layers of Ag NP ink, with particle size of ~20-50 nm, were gravure printed on a paper from Mitsubishi (NB-RC3GR120) to fabricate the SERS substrate by employing a laboratory scale gravure press (K-Printing Proofer from Testing Machines Inc.). The paper based printed substrate consisted of a row of 1 cm by 1 cm squares. After depositing the Ag NP, the substrate was cured in a VWR 1320 temperature controlled oven for 20 minutes at 130 °C. A photograph of the gravure printed SERS substrate is shown in Fig. 1. Figure 2 shows the profilometry of double layers of printed NPs on paper. The thickness of the single and double layered films was measured as 2 µm and 3.5 µm, respectively using vertical scanning interferometry with a WYKO RSTplus optical profiler. Root mean square (RMS) value of roughness of the single layer of printed Ag, measured using Bruker contour WLI was 80 nm as shown in Fig 3 (a) and (b). III.
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(a)
(b) Figure 1. (a) Single layer and (b) double layers of gravure printed silver nano ink on PET.
Figure 2. Vertical scanning interferometry of double layers of Ag.
EXPERIMENTAL PROCEDURE
The experimental setup is shown in Fig. 4. The HgS test solutions were immobilized on the single and double layered SERS substrate using 50 ml spray bottles. A laser source in the near infrared region with a wavelength of 785 nm was employed to excite the test sample using a Raman probe (Inphotonics Inc.), with integration time of 3 seconds at 300 mW. The Raman spectra was obtained through the collection fiber of the Raman probe and using a spectrometer (QE 65000 Ocean Optics, 780 nm - 1100 nm). Spectra Suite software (Ocean Optics) was used to analyze the Raman signature spectra of the target molecules.
Figure 3. (a) Vertical scanning interferometry single layer of Ag (b) Roughness of single layer of Ag on paper
IV.
RESULTS AND DISCUSSION
The response of the single and double layered gravure printed SERS substrate toward HgS is shown in Fig. 5. Raman peaks related to HgS were observed at 249.13 cm-1, 279.37 cm-1 and 339.21 cm-1. The enhancement factor (EF) of a target molecule can be obtained by a comparison between the intensities of the Raman peaks attained in presence and absence of metallic NPs, under similar test conditions, and is mathematically calculated using [22]:
EF =
I SERS N SURF I RS NVOL
(1)
Where NVOL is the average number of scattered molecules for Raman Spectrum and NSURF is the average number of adsorbed molecule under SERS measuremnt. IRSis the intensity of Raman signal in absence of NPs and ISERS represents the intensity of Raman signal in presence of NPs. The SERS based response of the printed substrate demonstrated 5 orders of magnitude enhancement in the intensity of Raman spectra for HgS immobilized on Ag NP printed substrate when compared to that of HgS immobilized on bare paper substrate by using the enhancement factor (EF) formula given in Eq. (1).
Figure 5. Raman spectrum of 10 µM mercury sulfide obtained from one layer and two layers on printed Ag NPs vs. 50 mM mercury sulfide on bare paper.
Raman Fixture
Figure 6. Effect of bending on Intensity.
Laser source
Spectrometer
Figure 4. Experimental setup. The enhancement factors obtained can be attributed to the effect of plasmonic resonance, which is a phenomenon that is caused mainly due to interaction between light and the metallic NP [15], in SERS. The weak Raman spectrum is enhanced by several orders of magnitude due to the introduction of strong electromagnetic fields at the hotspots on the printed SERS substrate [15, 16]. Hotspots or nanogaps, are regions created between the NPs due to the agglomeration and non-uniformity of NPs, in the molecular scale [15,16, 21]. Studies on NPs have reported that a decrease in the NP size will in turn decrease the gap between the particles and hence increase the electromagnetic intensity which leads to enhancement of Raman signals [17].
In order to investigate the effect of bending on the intensity of Raman spectra enhancement, the HgS was spray coated on a printed SERS substrate and its Raman spectra was measured for various bending angles. As shown in Fig 6, the intensity of the Raman spectra is directly proportional to the angle of bending. Enhancements of 20 %, 100 % and 500% in the intensity of Raman spectra were observed when the flexible SERS substrate was bent at 10°, 40° and 70°, respectively when compared to the Raman intensity of the flat substrate. This relation is considered in the enhancement of plasma resonance. While bending the NPs come closer and increases the number of hot spots. This creates a higher electromagnetic field which can be attributed to the enhancement in the intensity of the Raman spectrum. IV
CONCLUSION
A novel SERS substrate was successfully fabricated by gravure printing Ag NP ink on paper (NB-RC3GR120). The capability of the SERS substrate for toxic heavy metal detection was examined. An enhancement factor of 5 orders of magnitude was obtained for the detection of HgS when compared to the Raman spectrum produced from HgS on bare paper. The effect of bending of the flexible paper based SERS substrate on the intensity of Raman spectrum was also
studied. When compared to flat SERS substrate, an enhancement of 500 % in the intensity of Raman spectra was obtained for a bending of 70°. Enhancements in the Raman intensity were observed through increasing the bending angle of the substrate. Further research is underway for the development of substrate for SERS on stretchable substrates using different printing methods to be used in hand-held SERS based systems for the detection of a wider range of biochemical sensing applications. ACKNOWLEDGMENT This work has been partially supported by the U.S. Army Grant Nos.WS911QY-07-1-0003 and W911NF-09-C-0135. REFERENCES [1]
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