Mobile Phone-Based Picomolar Detection of Tannic Acid on Nd2O3

Jul 5, 2019 - Surface plasmon-coupled emission (SPCE) has emerged as a potential sensing platform owing to its >50% fluorescence signal collection ...
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Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Mobile Phone-Based Picomolar Detection of Tannic Acid on Nd2O3 Nanorod−Metal Thin-Film Interfaces Seemesh Bhaskar† and Sai Sathish Ramamurthy*,† †

STAR Laboratory, Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam, Puttaparthi, Anantapur, Andhra Pradesh 515134, India

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S Supporting Information *

ABSTRACT: Surface plasmon-coupled emission (SPCE) has emerged as a potential sensing platform owing to its >50% fluorescence signal collection efficiency. Further advancements toward boosting the coupling efficiency can be achieved by relevant spacer and cavity engineering. Several composites of metal nanoparticles (NPs) have been used along with different templates such as low dimensional carbon substrates (1D, 2D, and 3D), ceramics, proteins, and DNA, to name a few. However, they fundamentally suffer from intrinsic parasitic losses in metals and require nonzero nanogaps between them and the metal thin film for hot-spot generation. Here, we report the first-time application and significance of high refractive index (HRI) dielectric, rare earth, biocompatible Nd2O3 NPs as salient spacers to achieve template-free and metal NP-free, 118-fold emission enhancements in SPCE platform using a simple optical setup. The primary focus is on the effect of volume and size of nanoenvironment on the coupling of Nd2O3 nanorods with silver (Ag) thin film. In addition to this, we report a new cavity format as pseudo-MDHD (metal−dielectric−high refractive index dielectric) framework analogous to MDM (metal−dielectric−metal). This study also elaborates on the importance of Mie resonances and resonant light scattering in analyzing the emission enhancements obtained using spacer, cavity, and extended cavity interfaces. This work also demonstrates the first-time utility of cost-effective smartphone based SPCE studies for monitoring tannic acid (TA), a hazardous chemical in environmental water, at picomolar limit of detection (LOD) using HRI dielectric Nd2O3 nanorods. KEYWORDS: surface plasmon-coupled emission, fluorescence enhancements, high refractive index materials, dielectric nanoparticles, spacer, cavity, extended cavity, tannic acid, limit of detection

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INTRODUCTION Fluorescence spectroscopy has found widespread use in different applications: biological and chemical sensing,1 surface-enhanced Raman scattering (SERS),2 immunoassays,3 forensics,4 single molecule detection,5 to name a few. Despite this, conventional fluorescence spectroscopy has limitations: low signal collection efficiency ( 1.6 results in dwindled E-field intensity.19 As a result of this study, we present an optimum external environment using PVA (RI = 1.47) to achieve augmented coupling and E-field localization in the hot-spots. Consequently, the role of different architectures (interfaces) is indispensable for better elucidation of the enhancements summarized in Table 1. The spacer interface, as discussed earlier, is constituted with a 30 nm (using 1% PVA) or 60 nm (using 2% PVA) thin film doped with Nd2O3 NRs on top of Ag thin film, following which a 30 nm Rh6G doped PVA is mounted. This results in the summative film thickness of 60 nm (for 1% PVA) or 90 nm (for 2% PVA) and differentiated polymer encapsulated volumes. This, in turn, presents altered emission enhancements for the two different spacers. SPCE enhancements increased from 73-fold (1% PVA) to 118-fold (2% PVA) for 0.1 wt % Nd2O3 NRs. The increase in emission enhancements is further aided by reduced crystallinity of the hybrid film in case of 2% PVA spacer. On the contrary, for 1 wt % Nd2O3 NRs, the emission enhancements decreased from 83fold (1% PVA) to 61-fold (2% PVA) on account of increased crystallinity of 2% PVA spacer. In cavity interface, 30 nm (using 1% PVA) or 60 nm (using 2% PVA) thin film, doped with a mixture of Rh6G and Nd2O3 NRs, is spin coated on top of Ag thin film. As a consequence, this presents a summative thickness of 30 or 60 nm PVA film once again with differentiated polymer encapsulated volumes for 1 and 2% PVA. Hence, once again an increase in the emission enhancements from 61-fold (1% PVA) to 89-fold (2% PVA) for 0.1 wt % Nd2O3 NRs is encountered. Also, a decrease from 53-fold (1% PVA) to 42-fold (2% PVA) for 1 wt % Nd2O3 NRs doping is justifiable on similar lines. Extended cavity is inverse of spacer architecture, with the top layer containing Nd2O3 NRs doped in 1% PVA (30 nm) and 2% PVA (60 nm) being directly exposed to the laser light. Furthermore, since in both these cases Nd2O3 NRs extend out of the PVA film, an increased back scattering of the laser beam is anticipated. In line with the understanding and crystallinity pattern, we encounter nearly equal emission enhancements: 54-fold (1% PVA) and 55-fold (2% PVA) for 0.1 wt % Nd2O3 NRs. Also a decrease in SPCE enhancements is encountered for 1 wt % Nd2O3 NRs: 50-fold (1% PVA) to 20-fold (2% PVA). The decreased enhancements here are also attributed to the Nd2O3 NRs positioned at an increased distance of 30 nm from the Ag thin film. Modulated emission enhancements can be envisaged with the use of different film thickness. However, we have limited our studies to 30 and 60 nm PVA thin films, as increased thickness would result in multiple emissions at different angles with altered p-and s-polarization, confounding the analysis.64 On the basis of the architectural and interfacial design of the polymer encapsulated volumes, another important interpretation can be obtained from Table 1. Comparison of emission enhancements of the hybrid materials at a particular PVA film thickness, across the three interfaces, at a constant wt % of Nd2O3 NRs presents the following: for 0.1 wt % Nd2O3 NRs in 1% PVA the emission enhancements decreases from 73- to 61to 54-fold, as we move from spacer, to cavity, to extended cavity interface. Similar trends are observed for other weight percentages of Nd2O3 NRs and PVA. It is important to note here that differentiated polymer encapsulated volumes serve as H

DOI: 10.1021/acsanm.9b00987 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials

Figure 5. CIE chromaticity diagram presenting emission from Rh6G, with 0.1 wt % Nd2O3 NRs in spacer, cavity, and extended cavity interfaces in (a) 1 wt % and (b) 2 wt % PVA (left). Corresponding shade cards with details of emission enhancements (right).

Figure S12 presents a comparative analysis of the polarization % obtained from 0.1 and 1 wt % of Nd2O3 NRs in 1% and 2% PVA encapsulated matrix. Directional p-polarized emission obtained for all the samples distinctly captures the characteristic feature of the SPCE phenomenon. Mobile Phone-Based SPCE Analysis. In addition to the conventional detector (Ocean Optics spectrometer), we have carried out studies using Nd2O3 NRs as substrates on a userfriendly and cost-effective smartphone based SPCE platform.13,14,17 The SPCE enhancements (obtained using dielectric Nd2O3 NRs) resulted in different color of molecular emission depending on the interface designed and were captured using the cellphone camera. Figure 5 presents the modulation in visible color of emission from the emitter dipoles using CIE chromaticity plots. Color matching function of the human eye is defined by xyY values based on Commission Internationale de I’Eclairge, 1931 color space.66 We see that the CIE plot shows a direct correlation between the three interfaces used and the emission color (Rh6G emission at 560 nm), as depicted by the shade cards (Figure 5 right). The tristimulus values X, Y, and Z described elsewhere66 are normalized to obtain x, y, and z values. Furthermore, these values are plotted in 1931 CIE chromaticity diagram:14 x=

X X+Y+Z

(3)

y=

Y X+Y+Z

(4)

z=

Z X+Y+Z

(5)

Figure 5a and b depict modulation of visible color and Rh6G emission with 0.1 wt % Nd2O3 NRs doped in 1 and 2% PVA in spacer, cavity, and extended cavity interfaces. In Figure 5a and b, we observe that the emission wavelength for 0.1 wt % Nd2O3 NRs transitions from deep red to yellow, through orange color, in spacer, cavity, and extended cavity. This shift in color is in direct correlation with the interfaces used. On comparison of the SPCE spectra for the different interfaces, it is noted that the λmax (wavelength at which maximum emission is observed) is ∼566 nm for spacer and cavity interfaces, while it gets blue-shifted (hypsochromic shift) for extended cavity interface appearing at ∼558 nm. Hence, the color of the emission is shifted toward the green region for extended cavity interface in the CIE chromaticity diagram. Further, a small shift in the color of emission toward the yellow region is observed for cavity with regard to the spacer interface. This can be explained by taking in to account the emission pattern of Rh6G toward the higher wavelengths. From the SPCE spectra for spacer and cavity interface, it is noticed that the emission counts for spacer is more than cavity at higher wavelengths. For instance, for 0.1 wt % Nd2O3 NRs doped in 1% PVA, it is observed that the emission counts are 13541 and 10489 (a.u.) for spacer and cavity interfaces at 600 nm. This trend is similar for other samples as well. Therefore, owing to more red emission in spacer interface, we observe a shift toward red color of CIE chromaticity diagram compared to cavity interface. An equivalent conclusion can be drawn from the I

DOI: 10.1021/acsanm.9b00987 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials

increase in TA concentration (from 100 fM to 100 pM), possibly on account of amplified hot-spots. In the previous section on polymer encapsulated volume contribution toward enhancements, we observed that the enhancements dwindled fundamentally due to protrusion and agglomeration of larger sized Nd2O3 NRs outside the PVA thin film, resulting in back scattering. In this context, the amplification of emission enhancements observed with Nd2O3 NRs−TA hybrid is reasonable on account of increased dispersion of Nd2O3 NRs, aiding significant coupling and forward scattering. Figure 6 [left y-axis] clearly shows that negligible concentrations of TA from 1fM to 100 fM did not alter the emission properties of pristine Nd2O3 NRs and, hence, limited TA detection to 10pM. Furthermore, as seen in the case of pristine TA, here too the emission enhancements showed a downturn with increasing TA concentration. This again is due to increased agglomeration at higher TA concentrations. To replace expensive spectrometers with economically viable and portable instruments for monitoring water quality, we have adopted a cellphone camera as the detector to capture the directional emission form Nd2O3 NRs−TA hybrid interface. This is of utmost relevance when we consider TA sensing in environmental water bodies used for drinking purpose.44 Similar to the previous reports on luminosity values in SPCE experimentation,13 Figure 6 presents an excellent correlation of luminosity plot [right y-axis] with the enhancements presented in the left y-axis of the same figure. The corresponding gray scale shade cards are captured in Figure 6 [bottom], and the SPCE intensity plots along with their corresponding angular distribution plots are presented in Figures S14−S16. The comparative percentage polarization depicting predominance of p-polarized emission is shown in Figure S17. This is the first-time monitoring of TA at picomolar levels using mobile phone based SPCE platform with a range of 10 pM to 100 μM. However, the intricate relationship between the fluorescence emission intensities and range of TA concentrations mandates the use of two different study ranges, involving different TA concentrations. (i) For 10 nM to 100 μM: due to linear correlation of fluorescence emission signal with concentrations in this range, the detection of TA is straightforward. (ii) For 1pM to 10 nM: in this range, the fluorescence emission enhancements are larger than other TA concentrations that are either greater than 10 nM or lower than 1 pM. As a result of this modulation, the presence of TA in this range of concentration can once again be distinguished from samples having TA concentrations higher than 10 nM or lower than 1 pM. This ensures that the detection of TA in unknown samples can indirectly be achieved using both the detection ranges, a red background for semiquantitative detection and a green background for concentrations for quantitative detection. As explained earlier, TA in drinking water is toxic to humans as it can form halo-compounds and hence necessitates monitoring of water bodies. In this regard, further validation of the SPCE substrates was carried out in drinking water samples, spiked with 1 mM, 100 pM, and 100 nM concentrations of TA. This was mixed with 0.1 wt % of Nd2O3 NRs and studied on the SPCE platform. The obtained results are shown in Figure 6 [left y-axis], with results obtained for the spiked samples as well. The emission enhancements obtained were 120-, 150-, and 94-fold for 1 mM, 100 pM, and 100 nM TA concentration with a standard deviation of (±) 5% (Figure S18) for triplicate measurements (captured in the Supporting Information for better clarity of Figure 6) and presented good reliability and

shade cards as well. These results demonstrate the effectiveness of cell phone-based SPCE platform in analyzing the fluorescence signal using CIE chromaticity diagram and distinguishing the three major interfaces adopted in the plasmonic studies. Mobile Phone-Based Detection of Tannic Acid (TA) at Picomolar Concentration. The enhanced electric field confinement obtained by means of 0.1 wt % Nd2O3 NRs in 2% PVA was used for detection of environmentally hazardous organic pollutant, TA. Earlier reports suggest a strong affinity of TA for Nd2O3, which results in its surface adsorption and breakdown of Nd2O3 to smaller NPs. This phenomenon was explored on Nd2O3 NRs−Ag thin film hybrid interface, and the results are outlined in Figure 6. As seen in Figure 6 blank, in

Figure 6. Left y-axis: modulation in plasmon-coupled emission enhancements with increasing TA concentration (red stars indicate spiked samples). Right y-axis: luminosity plot presenting alteration in the emission intensity from Rh6G with increase in TA concentration. Bottom: gray scale shade card for different concentrations of TA.

the absence of TA and Nd2O3 NRs, an 11-fold emission enhancement was obtained [left y-axis]. TA in the absence of Nd2O3 NRs, in spacer interface, showed a marginal increase in emission enhancements of RhG6 from 11- to 23-fold with increasing concentration (from 1fM to 100 pM). As reported earlier, this is on account of π−π interaction of aromatic rings of TA and RhG6, which result in an orientation of dipole moment of Rh6G molecules.67 However, a decline in emission enhancements of pristine TA from 23- to 17-fold with further increase in its concentration from 10pM to 100 μM was observed. This is attributed to the well-known coagulation of TA in PVA matrix,68 which compromises the π−π stacking between TA and Rh6G. TA upon addition to Nd2O3 NRs is strongly adsorbed on the nanoparticles surface. In addition, TA is known to alter the particle surface charge of Nd2O3 particles on account of its reducing and stabilizing properties.46 Therefore, we see an augmentation of emission enhancements for Nd2O3 NRs−TA hybrid from 118-fold to 163-fold with an J

DOI: 10.1021/acsanm.9b00987 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials

variety of structural morphologies (eg, rod-shape, cube-shape, star-shape etc.) contributing different amplitude of hot-spots in SPCE platform remain unexplored. We strongly believe that this low-cost mobile phone-based TA sensing platform obtained using HRI dielectric NPs opens a new window for exploring the other potential dielectric NPs in SPCE platform with newer prospects to understand the physio-chemical interactions at nanodimensions.

reproducibility. Subsequently, the recovery studies were performed for drinking water sample. As the drinking water sample presented about 118-fold emission enhancements, based on our previous results, we conclude that the sample contained