Plasmonics DOI 10.1007/s11468-017-0538-9
Low-Cost Plasmonic Carbon Spacer for Surface Plasmon-Coupled Emission Enhancements and Ethanol Detection: a Smartphone Approach Pradeep Kumar Badiya 1 & Venkatesh Srinivasan 1 & Sai Prasad Naik 1 & Bebeto Rai 1 & Narendra Reddy 2 & S Prathap Chandran 1 & V Sai Muthukumar 3 & Muralikrishna Molli 3 & Sai Sathish Ramamurthy 1
Received: 26 December 2016 / Accepted: 3 February 2017 # Springer Science+Business Media New York 2017
Abstract Surface plasmon-coupled emission (SPCE) has led to significant advancements in analytical techniques on account of its unique characteristics that include highly polarized photon-sorting ability. In this study, we report the use of a low-cost activated carbon as a plasmonic spacer in the SPCE substrate for achieving 30-fold enhancement in fluorescence emission. We extend the use of this spacer in the presence of Rhodamine B Base, a lactone dye as the sensing material for smartphone-based ethanol detection on the SPCE platform. Ethanol detection from 1 to 6% concentration highlights the potential use of this technique in monitoring fermentation processes. Keywords Ethanol sensing . Surface plasmon-coupled emission . Activated carbon . Fluorescence enhancement . Plasmonic spacer engineering
Abbreviations SPCE Surface plasmon-coupled emission AC Activated carbon RhBB Rhodamine B Base * Sai Sathish Ramamurthy
[email protected] 1
Plasmonics Laboratory, Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam, Anantapur, Andhra Pradesh 515134, India
2
Centre for Incubation Innovation Research and Consultancy, Jyothy Institute of Technology, Thathaguni Post, Bengaluru 560082, India
3
Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam, Anantapur 515134, Andhra Pradesh, India
PMMA La
Poly(methyl methacrylate) In-plane lattice constant
Introduction Fluorescence spectroscopy is an analytical tool widely used in medicine, chemistry, and biochemistry applications that include determination of analyte concentration in fermentations, immunoassays, protein microarrays, and DNA detection [1–5]. However, the isotopic nature of fluorescence results in the observance of only 1% of emission, limiting its application in sensing technologies [6]. In this context, the need for superior analytical sensitivity and enhanced limit of detection has been addressed by the introduction of a novel approach called surface plasmon-coupled emission (SPCE) [7]. This technique enhances the collection of emission from a radiating dipole using a near-field approach. The fluorophore in this platform is positioned adjacent to a thin metal film (on a dielectric substrate) resulting in an enhanced, highly polarized, and directional fluorescence output [8]. The distinctive features of SPCE are as follows: (i) significant background suppression resulting from the use of a small sample volume and on account of SPCE being a near-field phenomenon (within 200 nm from the metal film), (ii) ≥10-fold fluorescence enhancement on coupling of 50% of the emitted fluorescence with the surface plasmons of the metal thin film, (iii) amplified collection efficiency resulting from directionality and p-polarization of SPCE emission, and (iv) spectral resolution of infinitesimal interrogation volumes. These properties have resulted in the translational application of SPCE platform in the detection of single molecules, protein monolayers, lifetime-based ultrafast sensing applications, and study of complex sample matrices: muscle and whole blood assay [9–13].
Plasmonics
In addition to these SPCE properties, the focus of our research so far has been to avail low-cost sensing instrumentation. However, the approach requires both low-cost plasmonic substrates and low-cost detection assembly as part of the SPCE platform. This resulted in the design of a low-cost, 1-min cavity engineering protocol [14] that has significantly improved the enhancements in fluorescence emission than conventional SPCE technique. We have extended the use of this simple technique for studying tunable fluorescence enhancements with a wide variety of nanomaterials that span metals, ceramics, lowdimensional carbon substrates, composites, and biomaterials [15]. The highlight of these studies has been the neoteric advancement that facilitates the use of mobile phone camera as a detector in the SPCE platform coupled with the design of novel architectures accomplished on the SPCE substrate. Blending the cavity, spacer, and qubit engineering along with the Commission Internationale de leclairge ( CIE) color space mapping, we have opened the doors to next-generation plasmonic platform by achieving, for the first time, a 1000-fold postulated fluorescence enhancement and have its translational application in picomolar sensing of coronary heart disease marker LpPLA2, femtomolar sensing of an organic dye, attomolar DNA detection [5], correlation of Purcell factor with SPCE enhancements [14, 16], and evaluation of the nature of (i) organic dye–nanomaterial [14, 17] and (ii) organic dye–biomaterial interactions [5]. In our recent work, we have designed the use of silver-coated carbon nanotubes as cavity material on SPCE substrates to achieve attomolar dopamine detection [14]. The significance of these developments was to obtain fluorescence enhancements [17] with economically viable substrate engineering protocols and to use an Android application in hand with a mobile phone camera to capture the CIE color space value of the directional plasmon-coupled emission [15, 18]. As an extension to the ongoing effort to avail low-cost plasmonic substrates, we have utilized activated carbon (AC) as a potential spacer material in the current study. AC is a microporous form of carbon, with large surface area and high porosity structure. These carbons are produced in bulk quantities from a number of raw materials, particularly agroindustrial by-products like walnut shells, coconut shells, and olive stones [19–22]. The by-products are frequently regarded as agro-waste raising disposal issues among environmentalists. Conversion of these agricultural wastes into ACs and their utilization in industries has been a feasible solution to this environmental concern. Increasing environmental pollution has seen an increased demand for ACs on account of their wide application in catalysis, remediation of organic pollutants from potable water, recovery of solvents, and separation of gases, to name a few [23]. Owing to its low-cost and high adsorption capacity, AC has been commonly used in chemical, pharmaceutical, and food industries [24]. In the present study, we have utilized AC as a spacer material in the SPCE substrate along with Rhodamine B Base (RhBB) as the fluorophore, towards detection of ethanol.
Materials and Methods Phosphorous-free activated charcoal was used as the carbon spacer material. Raman spectroscopy (DXR-Raman microscopy, Thermo Scientific) for the samples was carried out with laser excitation wavelength of 780 nm and 4 mW power. FT-IR spectroscopic study of the dried AC sample was carried out with an ATR add-on (Thermo Scientific Nicolet iS10). Microscopic images of the activated carbon materials were obtained using scanning electron microscope (ZEISS Gemini). Specific surface area of the activated carbon materials was determined by N2 adsorption–desorption method carried out at −193 °C. NOVA 1000 Quanta chrome highspeed gas sorption analyzer instrument was used to measure the specific surface area. The textural, porosity properties and pore size distribution of the carbon materials were also studied. Preparation of Spacer Layer Fifty-nanometer thick silver thin films coated on Pyrex glass slides were procured from EMF Corp, USA and used as the silver substrate. AC was dispersed in 1% poly(methyl methacrylate) (PMMA) polymer prepared in chloroform. Following this, a thin layer of RhBB in 1% PMMA was spin coated at 4500 rpm for 45 s, thus forming a double PMMA layer with RhBB as the top layer. The experiments for ethanol detection were carried out using the same architecture. Surface Plasmon-Coupled Emission Measurements The substrates were irradiated in the reverse Kretschmann (RK) arrangement with a p-polarized 532 nm c.w. laser (5– 50 mW, TE cooled module). The substrates were attached to the transparent hemi-cylindrical crystal prism with an index matching fluid and positioned onto a rotating stage. In the RK geometry, the sample was excited from the air side, free space (FS) side and the SPCE emission was detected from prism side (SPCE side). The fiber was fixed to a rotating arm that covered 0°–90° and 270°–360° to record the emission at different angles. A 550-nm long pass filer was placed in front of the collection fiber and the emission was filtered and recorded using Ocean Optics USB 2000+ spectrometer. A polarizer was placed in front of the filter either horizontally or vertically for recording the polarization of the emission fluorescence spectrum.
Results and Discussion The nitrogen adsorption–desorption isotherm (Fig. 1a) was used to obtain specific surface area (SBET) (using BET equation), total pore volume (Vtot), and pore diameter (Dp). AC has
Plasmonics
specific surface area and is microporous in nature [25]. SEM micrograph of AC (Fig. 1b) clearly presents a microstructural surface that is layered, uneven, dented, and highly porous in nature [21]. X-ray diffraction pattern for AC (Fig. 3c) shows the appearance of sharp peaks at 2θ = 24.375° and 43.275° which corresponds to disordered graphitic (002) plane [26]. Since AC is a complex material, its surface chemistry is governed by the presence of the heteroatoms like phosphorus, sulfur, hydrogen, oxygen, nitrogen, and other halogens bonded to the edges of the carbon [21, 25]. Structural chemistry of the carbon matrix was determined by FT-IR spectroscopy. FTIR spectrum showed a wide transmittance band at 3200– 3600 cm−1 with a maximum at about 3420–3440 cm−1 that corresponds to O–H functional group which in turn confirms the existence of hydroxide bonded to carbon (Fig. 2a). The presence of aromatic rings was confirmed by a strong peak observed at 1600–1580 cm−1 (characteristic C–C vibrations). A broad band at 1000–1300 cm−1 can be correlated to the
Fig. 1 a BET isotherm of N2 adsorption on activated carbon (AC) at −193 °C. b SEM micrograph showing microstructural aspects of surface morphology of AC. c XRD patterns showing prominent peaks at 2θ = 24.375° and 43.275° conforming to graphene-like structure of AC
SBET of 1700.8 m2/g, Vtot of 1.07 cm3/g, and Dp of 2.53 nm. From the BET results, it is very evident that the AC has a large
Fig. 2 a FT-IR spectrum of AC showing characteristic peaks. b Raman spectrum of AC showing D and G bands at 1290 and 1600 cm−1, respectively
Plasmonics Fig. 3 a Emission spectrum observed in free space (FS) and SPCE sides of the prism in the absence and presence of spacer/ cavity. b Plasmon-coupled emission enhancements achieved with AC in cavity and spacer architectures, compared to conventional SPCE (blank). c) Intensity of the p and s polarized emissions. d) Angular emission of plasmon-coupled emission, in cavity and spacer configurations
stretching band of C–O associated with oxidized carbons and has been assigned to C–O stretching as in carboxylic acids, esters, alcohols, ethers, and phenol groups [27]. The Raman spectrum of the AC material was obtained by scanning the 500 to 3500 cm−1 region (Fig. 2b). Characteristic D and G bands were observed in the spectrum indicating the graphitic nature of the AC. The peak intensity of D band is more than the G band, suggestive of defects. The La, in-plane lattice constant , was calculated to be 165 ± 5 nm, indicating a large crystallite size. Our studies on the SPCE platform present the use of AC as both spacer and cavity material. We have evaluated the plasmonic coupling efficiency in both these architectures by studying the plasmon-coupled fluorescence enhancements (Fig. 3a). AC yielded 14-fold and 31-fold enhancements in fluorescence signal as cavity and spacer material (Fig. 3b). Earlier, we had reported a 40-fold enhancement in fluorescence emission with the use of exfoliated graphene as spacer. In this study, the SPCE enhancement obtained with AC spacer is comparable to exfoliated graphene, confirming that AC is graphitic in nature, in line with the Raman bands [28]. Plasmon-coupling was confirmed by observing the emissions that were predominantly p-polarized (spacer 96% and cavity 91%) (Fig. 3c). The plasmon-coupled emission was observed at 55° and 56° for spacer and cavity (Fig. 3d), close to the
emission obtained with the graphene spacer at 53°. The earlier study with few layered graphene and exfoliated graphene resulted in 7-fold and 40-fold enhancements corresponding to La values: 70 ± 10 and 360 ± 10 nm [28]. The 31-fold fluorescence enhancement obtained with the AC spacer correlates well with its La value of 165 ± 5 nm. The crystallite size plays an important role in obtaining significant fluorescence enhancements. We have utilized this 31-fold increase in fluorescence emission obtained using the AC spacer engineering (vis-a-vis conventional SPCE substrate) for ethanol detection in fermentation broth. Online ethanol sensors are particularly important for monitoring fermentation processes in fuel and food industries, safety driving, and environmental control [29]. Currently, chromatographic [30], enzymatic [18], and optical [31, 32] measurements have been used for alcohol detection. Since we intend to design a real-time, in situ ethanol monitoring sensor, we have utilized RhBB, a solvatochromic dye. In this context, we have extended our prior experience in mobile phone-based plasmonics technology to develop a low-cost smartphone-based ethanol sensor. Figure 4a depicts the standard curve obtained for varying concentrations of ethanol from 1 to 6%. Figure 4b, c presents the chromaticity plot of plasmon-coupled emission and color shade card for different ethanol concentrations. There are several ethanol sensors
Plasmonics
opens the doors for the replacement of expensive materials in sensing applications. Acknowledgements S.S.R. and P.K.B acknowledge the support from DBT-Ramalingaswamy fellowship (102/IFD/SAN/776/2015-16), DSTFast Track, and UGC-BSR fellowship, Govt. of India. S.P.C acknowledges financial support from DST Inspire Faculty Award. Authors thank the Department of Physics, Sri Sathya Sai Institute of Higher Learning, for providing access to the X-ray diffraction and Raman spectroscopic facility purchased under financial support from DST-FIST. Guidance from Bhagawan Sri Sathya Sai Baba is also gratefully acknowledged. Compliance with Ethical Standards Conflict of Interest The authors declare that they have no conflict of interest.
References 1.
2.
3.
4. Fig. 4 a Modulation of plasmon-coupled emission intensity with ethanol concentration with AC spacer. b CIE color space plot presenting a shift in emission with change in ethanol concentration. c CIE color shade card as a quick calibration strip for sensing ethanol between 1 and 6%
based on semiconductors, electromotive force of fuel cells, optical property, and bioelectronic response of enzymes [33–38]. It is worthy to mention here that this study is the first of its kind to develop a mobile phone-based sensing application for ethanol ranging from 1 to 6% concentration on the SPCE platform. Further studies are underway to improve the accuracy and limit of detection for ethanol sensing and to extend its use in other scenarios.
5. 6.
7.
8.
9.
10.
Conclusions We have successfully demonstrated the use of AC as a spacer and cavity material on the SPCE platform. The results obtained were comparable with that of previously reported graphene spacers whose fabrication is time consuming, cumbersome, labor intensive, and hence, financially heavy. Use of AC as a plasmonic spacer thus provides an economically viable alternative, especially for SPCE applications. A nascent model for sensing ethanol with AC spacers developed in this study
11.
12.
13.
Ge X, Kostov Y, Rao G (2005) Low-cost noninvasive optical CO2 sensing system for fermentation and cell culture. Biotechnol Bioeng 89:329–334 Ge X, Hanson M, Shen H, Kostov Y, Brorson KA, Frey DD, Moreira AR, Rao G (2006) Validation of an optical sensor-based high-throughput bioreactor system for mammalian cell culture. J Biotechnol 12:293–306 Tanaka G, Funabashi H, Mie M, Kobatake E (2006) Fabrication of an antibody microwell array with self-adhering antibody binding protein. Anal Biochem 350:298–303 Guo XQ, Castellano FN, Li L, Lakowicz JR (1998) Use of a longlifetime Re(I) complex in fluorescence polarization immunoassays of high-molecular-weight analytes. Anal Chem 70:632–637 Kricka LJ (2002) Stains, labels and detection strategies for nucleic acids assays. Ann Clin Biochem 39:114–129 Van Orden A, Machara NP, Goodwin PM, Keller RA (1998) Single-molecule identification in flowing sample streams by fluorescence burst size and intraburst fluorescence decay rate. Anal Chem 70(7):14404–11451 Gryczynski I, Malicka J, Gryczynski Z, Lakowicz JR (2004) Radiative decay engineering 4. Experimental studies of surface plasmon-coupled directional emission. Anal Biochem 324:170– 182 Smith D, Kostov Y, Rao G (2008) Signal enhancement of surface plasmon-coupled directional emission by a conical mirror. Appl Opt 47:5229–5234 Kostov Y, Smith DS, Tolosa L, Rao G, Gryczynski I, Gryczynski Z, Malicka J, Lakowicz JR (2005) Directional surface plasmoncoupled emission from a 3 nm green fluorescent protein monolayer. Biotechnol Prog 21:1731–1735 Smith DS, Kostov Y, Rao G (2007) SPCE-based sensors: ultrafast oxygen sensing using surface plasmon-coupled emission from ruthenium probes. Sens Actuator B 127:432–440 Stefani D, Vasilev K, Bocchio N, Stoyanova N, Kreiter M (2005) Surface-plasmon-mediated single-molecule fluorescence through a thin metallic film. Phys Rev Lett 94(2):023005 Aslan K, Zhang Y, Geddes CD (2009) Surface plasmon-coupled fluorescence in the visible to near-infrared spectral regions using thin nickel films: application to whole blood assays. Anal Chem 81: 3801–3808 Borejdo J, Gryczynski Z, Calander N, Muthu P, Gryczynski I (2006) Application of surface plasmon coupled emission to study of muscle. Biophys J 91:2626–2635
Plasmonics 14.
15.
16.
17.
18.
19.
20.
21.
22. 23.
24.
25.
Venkatesh S, Ghajesh S, Ramamurthy SS (2015) 1-Minute spacer layer engineering for tunable enhancements in surface plasmoncoupled emission. Plasmonics 10:489–494 Srinivasan V, Vernekar DV, Jaiswal G, Jagadeesan D, Ramamurthy SS (2016) Earth abundant iron-rich N-doped graphene based spacer and cavity materials for surface plasmon-coupled emission enhancements. ACS Appl Mater Interfaces 8:12324–12329 Venkatesh S, Badiya PK, Ramamurthy SS (2016) Purcell factor based understanding of enhancements in surface plasmon-coupled emission with DNA architectures. Phys Chem Chem Phys 18(2): 681–684 Srinivasan V, Ramamurthy SS (2015) Purcell factor: a tunable metric for plasmon-coupled fluorescence emission enhancements in cermet nanocavities. J Phys Chem 120(5):2908–2913 Venkatesh S, Badiya PK, Ramamurthy SS (2015) Low-dimensional carbon spacers in surface plasmon-coupled emission with femtomolar sensitivity and 1000-fold fluorescence enhancements. Chem Commun 51:7809–1711 Badiya PK, Srinivasan V, Jayakumar TP, Ramamurthy SS (2016) Ag-CNT architectures for attomolar dopamine detection and 100fold fluorescence enhancements with cellphone-based surface plasmon-coupled emission platform. ChemPhysChem 17:2791– 2794 Rodriguez-Reinoso F, Molina-Sabio M, Gonzalez MT (1995) The use of steam and CO2 as activating agents in the preparation of activated carbons. Carbon 33:15–23 Hu Z, Srinivasan MP (1999) Preparation of high surface area activated carbons from coconut shell. Microporous and Mesoporous Mat 27:11–18 Hu Z, Vansant EF (1995) Carbon molecular sieves produced from walnut shell. Carbon 33:561–567 Ahmadpour A, Do DD (1997) The preparation of activated carbon from macadamia nutshell by chemical activation. Carbon 35:1723– 1732 Yacob AR, Majid ZA, Dasril RS, Inderan V (2008) Comparison of various sources of high surface area carbon prepared by different types of activation. Malaysian J Anal Sci 12:264–271 Hayashi JI, Uchibayashi M, Horikawa T, Muroyama K, Gomes VG (2002) Synthesizing activated carbons from resins by chemical activation with K2CO3. Carbon 40:2747–2752
26.
Yakout SM, El-Deen GS (2011) Characterization of activated carbon prepared by phosphoric acid activation of olive stones. Arab J Chem 9:S1155–S1162 27. Nautiyal P, Subramanian KA, Dastidar MG (2016) Adsorptive removal of dye using biochar derived from residual algae after in-situ transesterification: alternate use of waste of biodiesel industry. J Environ Manag 182:187–197 28. Zawadzki J (1989) Infrared spectroscopy in surface chemistry of carbons. Chem Phys Carbon 21:147–386 29. Mulpur P, Podila R, Lingam K, Vemula SK, Ramamurthy SS, Kamisetti V, Rao AM (2013) Amplification of surface plasmon coupled emission from graphene–Ag hybrid films. J Phys Chem C 117:17205–17210 30. Nagamura T, Tanaka H, Matsumoto R (2010) High performance optical sensing of alcohol by guided wave mode geometry composed of two polymer layers. Sens Actuators B 146:253–259 31. Buttler T, Johansson KAJ, Gorton Lo GO, Marko-Varga GA (1993) On-line fermentation process monitoring of carbohydrates and ethanol using tangential-flow filtration and column liquid chromatography. Anal Chem 65:2628–2636 32. Azevedo AM, Prazeres DMF, Cabral JMS, Fonseca LP (2005) Ethanol biosensors based on alcohol oxidase. Biosens & Bioelectr 21:235–247 33. Petrova S, Kostov Y, Jeffris K, Rao G (2007) Optical ratiometric sensor for alcohol measurements. Anal Lett 40:715–727 34. Franke ME, Koplin TJ, Simon U (2006) Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter? Small 2:36–50 35. Bull DR, Harris GJ, Rashed AB (1993) A connectionist approach to fuel cell sensor array processing for gas discrimination. Sens Actuators B 15:51–161 36. Millet P, Michas A, Durand R (1996) A solid polymer electrolytebased ethanol gas sensor. J Appl Electrochem 26:933–937 37. Mitsubayashi K, Matsunaga H, Nishio G, Toda S, Nakanishi Y (2005) Bioelectronic sniffers for ethanol and acetaldehyde in breath air after drinking. Biosens Bioelectron 20:1573–1579 38. Blum P, Mohr GJ, Matern K, Reichert J, Spichiger-Keller UE (2001) Optical ethanol sensor using lipophilic Reichardt’s dyes in polymer membranes. Anal Chim Acta 432:269–275
