Purcell Factor: A Tunable Metric for Plasmon-Coupled Fluorescence

Dec 2, 2015 - We demonstrate an important approach to correlate Purcell factor (PF) and surface plasmon-coupled emission (SPCE) enhancements with the ...
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Purcell Factor: A Tunable Metric for Plasmon-Coupled Fluorescence Emission Enhancements in Cermet Nanocavities Venkatesh Srinivasan and Sai Sathish Ramamurthy* Plasmonics Laboratory, Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam, Puttaparthi, Anantapur, Andhra Pradesh, India 515134 S Supporting Information *

ABSTRACT: We demonstrate an important approach to correlate Purcell factor (PF) and surface plasmon-coupled emission (SPCE) enhancements with the use of finitedifference time-domain (FDTD) simulations and timecorrelated single-photon counting (TCSPC) studies of a radiating dipole in cermet nanocavities. We observed >50-fold fluorescence enhancement with high directionality and polarization of Rhodamine 6G (Rh6G) emission trapped in the nanocavity created between the titanium-based ceramic nanoparticle and metallic silver thin film. Compositional variation with hybrid nanoparticles, TiC 0 N 1 (TiN), TiC0.5N0.5 (TiCN), and TiC1N0 (TiC), brought about enhanced PFs and tunable fluorescence enhancements that were used for mobile-phone-based detection of tryptophan with nanomolar sensitivity. We hope that this study opens the door to next-gen plasmonics with the ability to tune and enhance the hot-spot electromagnetic field intensity of alternative plasmonic materials, as hybrid synergy spacers in the SPCE platform. film to increase the photonic density of states and electromagnetic field intensity. This architecture has shown an increase in the electromagnetic field intensity, in hot spots between the nanoparticle and the silver thin film, resulting in augmented SPCE enhancements of Rh6G.11 In the current study we address the following important needs directed toward the development of the SPCE platform. i. Purcell Factor As an Experimental Metric. Light− matter interactions and the ability to tune plasmonic nanostructures have been exploited toward several applications.13,14 In addition to this, techniques based on coupling of spontaneous emission from a radiating dipole with the electromagnetic modes have been widely used in sensing and biomedical applications.15 In this regard, the effect of a cavity environment on the spontaneous emission is known as Purcell effect and is measured as Purcell factor (PF).16,17 At the moment, barring a few analytical techniques, PF has attracted considerable attention in plasmonics and metamaterials. One such Purcell effect under-researched technique is SPCE. To date, fluorescence signal enhancements in SPCE, using a nanomaterial-based spacer, have always been correlated with electromagnetic field enhancements, simulated using the FDTD approach.18,19 However, enhancements obtained in SPCE using silver thin films, with reverse Kretschmann excitation and observation of p-polarized angular emission on

1. INTRODUCTION In 2004, Lakowicz et al. reported the unique emission properties of fluorophores present near a metal thin film, arising on account of the coupling between the radiating dipole and the surface plasmons.1,2 Surface plasmon-coupled emission (SPCE) since then has been used for several applications that includes its photon-sorting ability3 and anisotropic fluorescence collection leading to 10−14-fold emission enhancements.4−6 SPCE-based fluorescence enhancement is defined as the increase in the intensity of directional emission compared to the free space isotropic emission. An important approach to improve the obtained fluorescence enhancements is to boost the coupling between the fluorophore and the surface plasmons.7 An earlier work utilized a monospacer layer of silver nanoparticles that resulted in fluorescence enhancements superior to conventional SPCE.7 Prior work from our lab has demonstrated neoteric advancements in SPCE spacer engineering technology with the use of plasma-etched polymer films,8 noble metal nanoparticles,9 semiconductor nanoparticles,10 and low-dimensional carbon allotropes to achieve femtomolar sensitivity and 1000-fold fluorescence enhancements.11 In a conventional SPCE substrate, the dye−polymer layer is spincoated onto a 50 nm silver/gold thin film, vapor deposited on a glass slide.4 An additional spacer layer could be engineered to lodge the nanomaterials between the dye and the metal thin film, in order to boost radiative coupling and in turn lessen nonradiative deactivation.6,12 Alternatively, the fluorophore can be sandwiched between the nanomaterials and the metal thin © XXXX American Chemical Society

Received: November 18, 2015

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The Journal of Physical Chemistry C the prism side, have not been evaluated and correlated with PF as an experimental metric across a variety of nanomaterials. ii. Alternative Plasmonic Materials as Spacers. The plasmonic properties of noble metals and carbon-based composites have been overexploited for numerous applications. Hence the need for alternate materials20 that can replace noble metals, with superior optical and plasmonic properties, has led to research in intermetallic-like compounds. Transition metal carbides and nitrides have been widely used for their unique properties especially in electrocatalysis 21 and fuel cell applications.22,23 Recently, titanium-based nanomaterials have been used for plasmonic applications.24 Specifically, titanium nitride (TiN) was reported to show plasmonic properties in visible and near-infrared wavelengths.25 The unique features of TiN with interesting optical properties, high stability toward corrosion, and temperature changes can hence be tapped for spacer engineering in SPCE substrates. In the present work, we treat the ceramic−dielectric−metal architecture as a cermet nanocavity and determine the PF and SPCE for varying compositions of the ceramic nanoparticles: TiC0N1 (TiN), TiC0.5N0.5 (TiCN), and TiC1N0 (TiC). Variation in the composition of the ceramic nanoparticles brought about tunable SPCE enhancements. Since the SPCE spacer layer engineering so far has made use of only metallic nanoparticles7,26 and carbon materials,11 we believe that hybrid synergy spacers designed through a judicious combination of alternative plasmonic materials will have a far reaching impact in the plasmonics arena. It is worthwhile to mention here that these are the first set of experiments related to the application of ceramic nanomaterials in the SPCE platform to achieve 51fold enhancement in plasmon-coupled emission, superior to our earlier spacer materials graphene12 and C6027 (40- and 30fold enhancements), in terms of both enhancements and the requirement of low-cost equipment. In addition to this, we have developed a short process time for substrate fabrication. A simple 1 min method to develop the dye overcoat layer (having the thin film containing the nanomaterial coated on top of the dye layer) results in the design of ceramic−dielectric−metal architectures. This in turn creates the cermet nanocavity with Rh6G trapped between titanium-based ceramic nanoparticle and metallic silver thin film. This study is in continuation of our earlier work11 that has shown the effect of Rh6G trapped between a carbon spacer and silver thin film on the SPCE enhancement of a fluorophore. We also report an enhanced level of tryptophan detection with the use of a mobile-phonebased SPCE platform.

Figure 1. Optical scheme of SPCE platform (top left). Hot spot formation at the Cermet nanocavity (top right). Plasmon-coupled fluorescence enhancements obtained with TiC/Ag, TiN/Ag, and TiCN/Ag nanocavities (bottom).

configuration with a 532 nm c.w. laser (5 mW), and the emission was passed through a 550 nm long-pass filter, before collecting into a fiber coupled to Ocean Optics USB 2000+ fiber optic spectrometer. The angularity and enhancements were measured with the help of a rotating stage. Polarization measurements were carried out by placing a polarizer between the prism and the collection fiber. In the case of the mobilephone-based SPCE platform, the mobile phone was mounted on the rotating stage with a 550 nm long-pass filter taped over the camera, and images of the fluorescence emission were captured from various angles. A commercially available Moto X running Android Lollipop 5.1 with 10MP camera was used for the collection of the emission images. An in-built android camera app was used to take snapshots of the angular emission. The images taken were processed with an android application “Color Grab” (available in Google Play store) that exported the corresponding values of the concerned variables to Commission Internationale de léclairage (CIE) xyY 1931 color space.30 2.3. SPR-Based Minimum Reflectivity Simulations. In order to validate the observed emission angles, simulations based on surface plasmon resonance (SPR) theory were carried out using TFCalc software (Software Spectra, OR) that gave the angle of minimum reflectivity. As reported earlier,31 the listed parameters were used as inputs during the simulations: PVA overcoats (ns = 1.50) on 50 nm silver thin film (ϵm = −14.06 + 0.45i) deposited on 0.7 mm BK7 glass (ng = 1.518) for the respective emission wavelength maxima corresponding to each nanocavity.2,4,5 2.4. TCSPC Studies. TCSPC studies were carried out to determine the time-resolved fluorescence decay profiles. The excitation source used was 490 nm pulsed LED. The emission intensities were counted by the Horiba Jobin Yvon TCSPC

2. EXPERIMENTAL SECTION 2.1. Fabrication of a Plasmonic Substrate. A 50 nm silver thin film coated on pyrex with a 5 nm silica top layer was purchased from EMF Corp, USA. A 1 mM concentration of Rh6G in 1% PMMA in chloroform was used to fabricate the 15 nm dye layer on the silver thin film as reported earlier.28 The dye overcoat layer was fabricated in 1 min, by spin coating the ceramic nanoparticles onto the dye layer. The ceramic nanomaterials of 1 mg/mL concentration in 1% polyvinyl alcohol (PVA) were used for fabricating the dye overcoat layer. These were spin-coated at 3000 rpm for 60 s.1 2.2. SPCE Measurements. Figure 1(top left) shows the optical scheme of the SPCE platform. The SPCE substrate was attached to a hemicylindrical prism using an index matching fluid (glycerol, n = 1.47) and mounted on a rotating stage.29 The substrates were illuminated in reverse Kretschmann B

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The Journal of Physical Chemistry C system, and the fluorescence decay was analyzed using IBHDAS V6.2. 2.5. FDTD Simulations. Commercially available Lumerical FDTD solutions software was used for FDTD simulations. We carried out a series of 3D FDTD simulations to study the orientation effect of a radiating dipole in cermet nanocavities and its effect on Purcell factor. Figure 1 (top right) shows the hot-spot formation at the cavity. All materials were built in the software using the reported properties.25,32−35 A point dipole source with single emission wavelength maxima was treated as the fluorophore that is placed between the ceramic nanomaterial and silver thin film to excite the transverse magnetic mode surface plasmon polariton wave. The distance between the ceramic nanomaterial and the silver thin film was changed to modify the cavity size with the radiating dipole present at the median. The hot spot intensity and PFs were extracted from the simulation result. The observed trend correlated well with that of the plasmon-coupled emission enhancements and experimental PFs.

3. RESULTS AND DISCUSSION 3.1. Enhancements. Figure 1 (bottom) shows the fluorescence enhancements obtained with different cermet nanocavities. The use of intermetallic-like compounds, TiC, TiCN, and TiN, resulted in fine-tuning the electromagnetic field intensity in the nanocavity and PF (see figure S1 in SI). While TiC is nonmetallic compared to TiN due to the presence of carbon, TiCN is expected to have hybrid properties of TiC and TiN to a certain extent.25 Interestingly, we found that the TiCN/Ag nanocavity showed amplified SPCE enhancements when compared to other cermets. Figure 2a presents the enhanced SPCE spectra obtained using the three nanocavities with different emission maxima (see Figure S2 in SI for SPCE and FS spectra in the absence of a nanocavity). We attribute this difference in the highly p-polarized (98%) plasmoncoupled emission peak wavelength to the presence of an interaction between Rh6G and the ceramic nanomaterial that alters the energy levels of the excited fluorophore.3,36 The TfCalc simulation of the surface plasmon resonance angle for Rh6G, based on the emission maxima in the presence of the nanocavities, correlated well with the observed fluorescence emission at different SPCE angles (Figure 2b). 3.2. Purcell Factor. We have also carried out TCSPC studies of Rh6G in cermet nanocavities. We found that when compared to the blank Rh6G in PVA on a glass slide the presence of nanocavities resulted in fluorescence lifetime reduction of Rh6G (Figure 2c). It is important to note here that the decrease in the fluorescence lifetime correlated well with the increase in SPCE enhancements. The fluorescence lifetime values were used to calculate PF.16,17 Figure 3a presents the correlation between the experimentally determined PFs (6.9, 8.3, and 9.8 for TiC/Ag, TiN/Ag, and TiCN/Ag nanocavities) and the obtained SPCE enhancements. FDTD simulations helped to calculate (theoretical) PFs for the nanocavities under study. These showed a similar trend as the experimental PFs. The variation seen in the case of TiC/Ag and TiCN/Ag (Figure 3b) might be attributed to the simulation’s all-inclusive inability, associated with the coupling interactions between the fluorophore and the nanomaterial. These interactions possibly alter the molecular energy levels, resulting in the variation of Rh6G emission maxima in the presence of the different nanocavities. Further, the experimentally determined PFs however showed a real-time

Figure 2. (a) Free space and plasmon-coupled fluorescence emission of Rh6G trapped in TiC/Ag, TiN/Ag, and TiCN/Ag nanocavities. (b) Plasmon resonance dips for TiCN/Ag, TiN/Ag, and TiC/Ag nanocavities. (c) Fluorescence decay of Rh6G in cermet nanocavities. Inset: Decay profile of Rh6G in PVA on glass.

correlation with the SPCE enhancements. The highlight of the current study has been to extend the intuitive correlation between PF and fluorescence emission to plasmon-coupled fluorescence emission enhancements obtained using the SPCE platform. Interestingly, we observed that the FDTD simulations showed negligible PFs for s-polarized emission in nanocavities when compared to that of p-polarized emission (see Table S1 in SI). In conventional SPCE, the perpendicularly oriented radiating dipoles are known to couple efficiently with the C

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perpendicular oriented fluorophores than those with the parallel orientation. In the current study, we extend the similar ability of the TiCN/Ag nanocavity to recognize the effect of aggregate formation on the SPCE intensity. In this regard, fluorescence-based techniques have been widely utilized in the recent past for protein- and peptide-based research. In particular, tryptophan is of great interest as it acts as an electron donor in photoinduced electron transfer reactions.40 Tryptophan’s role in the maintenance of the nitrogen balance in the human body and potential to be used to detect tumors41 is well-known. Fluorescence quenching by tryptophan on the plasmoncoupled emission of Rh6G trapped in the TiCN/Ag cermet nanocavity resulted in a rapid and sensitive mobile-phone-based detection of tryptophan. In this study, with the use of the SPCE platform, we report a detection limit of 10 nM of tryptophan (Figure 4a). Figure 4b shows the change in the CIE plot that corresponds to the decreasing concentration of tryptophan. This result is on par with the finest limit of detection (10 nM of tryptophan) achieved so far with the electrochemical use of gold nanoparticles/carbon nanotube film.42 However, the

Figure 3. (a) Correlation plot showing an increase in the SPCE enhancement with increasing PF. (b) Experimental and FDTD simulation-based PFs. (c) Polarization of free space emission in the presence of nanocavities in comparison with the blank. (d) Variation in theoretical PF with nanocavity size.

surface plasmons, as they emit electromagnetic radiation with the electric field perpendicular to the underlying metal surface, i.e., p-polarized.2 This results largely in p-polarized SPCE emission observed on the prism side. Hence the theoretical observation of negligible PFs for s-polarized emission is suggestive of the enhanced influence of the nanocavity on the radiating dipole oriented perpendicular to the silver surface than those fluorophores oriented parallel. This would alter the polarization of the fluorescence emission in the free space region. In line with our understanding, we found dissimilar polarization of Rh6G emission in the free space region using the three spacer materials. 55%, 63%, and 67% p-polarized emission in the free space was obtained in the presence of TiC/ Ag, TiN/Ag, and TiCN/Ag cermets (Figure 3c). However, in the absence of the nanocavities, 50% p-polarized emission was obtained in the free space region. Variation in p-polarized free space emission, between the nanocavities under study, can be attributed to the extent of dipole−dipole interaction: (i) the localized surface plasmon resonance dipole (of the ceramic nanoparticle) and (ii) the molecular dipole (of Rh6G), each of these are in turn affected by the propagating plasmons.37 We also believe that the change in the emission polarization can be attributed to the fano resonance between the LSPR of the ceramic nanomaterial and the propagating plasmons on the silver thin film.38,39 Further studies to better understand this phenomenon are under way. It is important to note that the order of increase in the p-polarized free space emission is equivalent to that of augmented PF and SPCE enhancement. Further, tuning of PF could also be achieved by varying the size of the nanocavity. Figure 3d presents the variation in theoretical PF with increasing distance between the ceramic nanoparticle and silver thin film. 3.3. Tryptophan Detection. In our earlier work using the SPCE platform, we studied the coupling efficiency of higherorder aggregates, formed by the interaction of the aromatic rings in the radiating dipole with that of graphene/carbon nanotubes,11 to evaluate the presence of weakly fluorescent aggregates. This was on account of the preferential, enhanced coupling efficiency of the graphene π-plasmons with the

Figure 4. (a) Plasmon-coupled emission quenching with increasing tryptophan concentrations. (b) CIE plot showing the change in emission color as perceived by the human eye, with increase in tryptophan concentration. D

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Emission from Molecular Multiplexes. Appl. Phys. Lett. 2009, 94, 223113−1−3. (4) Smith, D. S.; Kostov, Y.; Rao, G.; Gryczynski, I.; Malicka, J.; Gryczynski, Z.; Lakowicz, J. R. First Observation of Surface PlasmonCoupled Emission Due to LED Excitation. J. Fluoresc. 2005, 15, 895− 900. (5) Xie, T. T.; Liu, Q.; Cai, W. P.; Chen, Z.; Li, Y. Q. Surface Plasmon-Coupled Directional Emission Based on a ConformationalSwitching Signaling Aptamer. Chem. Commun. 2009, 22, 3190−3192. (6) Geddes, C. D.; Gryczynski, I.; Malicka, J.; Gryczynski, Z.; Lakowicz, J. R. Directional Surface Plasmon Coupled Emission. J. Fluoresc. 2004, 14, 119−123. (7) Chowdhury, M. H.; Ray, K.; Geddes, C. D.; Lakowicz, J. R. Use of Silver Nanoparticles to Enhance Surface Plasmon-Coupled Emission (SPCE). Chem. Phys. Lett. 2008, 452, 162−167. (8) Ramamurthy, S. S.; Kostov, Y.; Rao, G. Studies of SurfaceAdsorbed Fluorescently Labeled Casein and Concanavalin A Using Surface Plasmon-Coupled Emission. Plasmonics 2010, 5, 383−387. (9) Venkatesh, S.; Ghajesh, S.; Ramamurthy, S. S. 1-minute Spacer Layer Engineering for Tunable Enhancements in Surface PlasmonCoupled Emission. Plasmonics 2015, 10, 489−494. (10) Venkatesh, S.; Mandava, S.; Nayak, L.; Ramamurthy, S. S.; Neeleswar, S. Novel Synthesis of Nanoparticles for Enhancements in Surface Plasmon Coupled Emission. OSA Technical Digest 2014, M4A.44. (11) Venkatesh, S.; Badiya, P. K.; Ramamurthy, S. S. LowDimensional Carbon Spacers in Surface Plasmon-Coupled Emission with Femtomolar Sensitivity and 1000-Fold Fluorescence Enhancements. Chem. Commun. 2015, 51, 7809−7811. (12) Mulpur, P.; Podila, R.; Lingam, K.; Vemula, S. K.; Ramamurthy, S. S.; Kamisetti, V.; Rao, A. M. Amplification of Surface Plasmon Coupled Emission from Graphene-Ag Hybrid Films. J. Phys. Chem. C 2013, 117, 17205−17210. (13) Harutyunyan, H.; Martison, A. B. F.; Rosenmann, D.; Khorashad, L. K.; Besteiro, L. V.; Govorov, A. O.; Wiederrecht, G. P. Anomalous Ultrafast Dynamics of Hot Plasmonic Electrons in Nanostructures with Hot Spots. Nat. Nanotechnol. 2015, 10, 770−774. (14) Agarwal, G. S.; Harshawardhan, W. Inhibition and Enhancement of Two Photon Absorption. Phys. Rev. Lett. 1996, 77, 1039−1042. (15) Xia, F.; Wang, H.; Xaio, D.; Dubey, M.; Ramasubramaniam, A. Two-dimensional Materil Nanophotonics. Nat. Photonics 2014, 8, 899−907. (16) Purcell, E. M. Spontaneous Emission Probabilities at Radio Frequencies. Phys. Rev. 1946, 69, 681. (17) Unitt, D. C.; Bennett, A. J.; Atkinson, P.; Ritchie, D. A.; Shields, A. Polarization Control of Quantum Dot Single-Photon Sources via A Dipole-Dependent Purcell Effect. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 033318−1−4. (18) Chowdhury, M. H.; Pond, J.; Gray, S. K.; Lakowicz, J. R. Systematic Computational Study of the Effect of Silver Nanoparticle Dimers on the Coupled Emission from Nearby Fluorophores. J. Phys. Chem. C 2008, 112, 11236−11249. (19) Daniels, J. K.; Chumanov, G. Nanoparticle-Mirror Sandwich Substrates for Surface-Enhanced Raman Scattering. J. Phys. Chem. B 2005, 109, 17936−17942. (20) Naik, G. V.; Shalaev, V. M.; Boltasseva, A. Alternative Plasmonic Materials: Beyond Gold and Silver. Adv. Mater. 2013, 25, 3264−3294. (21) Ayats, M. R.; Garcia, G.; Pena, M. A.; Huerta, V. M. Titanium Carbide and Titanium Carbonitride Electrocatalyst Supports; Modifying Pt-Ti Interface Properties by Electrochemical Potential Cycling. J. Mater. Chem. A 2014, 2, 18786−18790. (22) Avasarala, B.; Haldar, P. Electrochemical Oxidation Behavior of Titanium Nitride Based Electrocatalysts Under PEM Fuel Cell Conditions. Electrochim. Acta 2010, 55, 9024−9034. (23) Ham, D. J.; Lee, J. S. Transition Metal Carbides and Nitrides as Electrode Materials for Low Temperature Fuel Cells. Energies 2009, 2, 873−899.

simple protocol and low-cost equipment adapted in the current study presents a facile approach for tryptophan detection.

4. CONCLUSIONS In this article, we demonstrate the use of cermet nanocavities for attaining SPCE signal enhancements. In contrast to all previous works, the present investigation outlines the correlation between PF and SPCE of emitters in the hot spots engineered using cermets and silver thin film. On the basis of our analysis, the effects of compositional variation and cavity thickness on the performance of a cermet nanocavity were investigated with an aim of enhancing the SPCE intensity and the PF tunability. We also have demonstrated the application of mobile-phone-based SPCE platform with a nanocavity-based substrate toward tryptophan sensing. This could pave the way for point-of-care fluorescence-based detection platforms. Further investigation of augmented hot spots, correlation studies on PFs, and fano resonances of radiating dipoles in the presence of different nanocavities and their corresponding SPCE enhancements is currently underway. We strongly believe that the high enhancements of plasmon-coupled emission and its correlation with PF and nanocavity size could be used as a simple method to determine the refractive index and polymer swelling behavior.43 In addition to the recently established relation between PF and nanoresonators,44 we hope that our proposed correlation opens up a practical approach for tunable near-field plasmonic devices and extends the use of PF in nanotechnology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11311. Simulation and polarization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge the support from DBT-Ramalingaswamy fellowship and UGC-BSR fellowship, Govt. of India. Special thanks to Prof. P. Ramamurthy, National Centre for Ultrafast Processes, University of Madras, for providing access to the time-correlated single-photon counting facility. Guidance from Bhagawan Sri Sathya Sai Baba is also gratefully acknowledged.

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REFERENCES

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