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Cellphone Monitoring of Multi-Qubit Emission Enhancements from Pd-Carbon Plasmonic Nanocavities in Tunable Coupling Regimes with Attomolar Sensitivity Venkatesh Srinivasan, Anupam Kumar Manne, Sai Gourang Patnaik, and Sai Sathish Ramamurthy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07445 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 20, 2016

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ACS Applied Materials & Interfaces

Cellphone Monitoring of Multi-Qubit Emission Enhancements from Pd-Carbon Plasmonic Nanocavities in Tunable Coupling Regimes with Attomolar Sensitivity Venkatesh Srinivasan, Anupam Kumar Manne‡, Sai Gourang Patnaik‡ and Sai Sathish Ramamurthy*. Plasmonics Laboratory, Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam, Puttaparthi, Anantapur, Andhra Pradesh, India - 515134. Keywords: Surface plasmon-coupled emission, Coupling regime, DNA detection, Cellphone, Palladium nanocomposites, Qubit.

Abstract:

We demonstrate for the first time, the tuning of qubit emission based on cavity engineering on plasmonic silver thin films. This tunable transition from weak to strong coupling regime in plasmon-coupled fluorescence platform was achieved with the use of palladium nanocomposites. In addition to our recently established correlation between Purcell factor and surface plasmoncoupled emission enhancements, we now show that the qubit-cavity environment experiences the

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Purcell effect, Casimir force, internal fano resonance and Rabi splitting. Finite-difference timedomain simulations and time correlated single photon counting studies helped probe the molecular structure of the radiating dipole, rhodamine-6G in palladium based nanocavities. The sensitivity of the qubit-cavity mode helped attain a DNA detection limit of 1 aM (attomolar) and multi-analyte sensing at picomolar concentration with the use of a smartphone camera and CIE color space. We believe that this low-cost technology will lay the groundwork for mobile phone based next-gen plasmonic sensing devices.

Introduction Bright single photon emitters are essential for quantum information systems.1 In this regard, spontaneous emission from two-level systems, single photon emitters such as quantum dots, organic molecules is widely used as the source for single photons.2 The rate limiting factors in achieving maximum emission intensity are low collection efficiency and low quantum yield. The electronic excited state of these emitters typically have a longer intrinsic lifetime (2-20 ns), limiting the number of photons emitted in a given period. However, spontaneous emission rates of radiating dipoles can be increased by placing the emitter in a cavity.3 This phenomenon called the Purcell effect is measured in terms of Purcell factor (PF).4 An alternate method is to modify the decay rate in order to achieve higher emission intensity, by exploiting the light-matter interaction. Metallic nanostructures supporting localized surface plasmons have been shown to increase the emission capability of quantum dots and organic dyes by manipulating the radiative decay pathway, resulting in shorter decay time.5 In this context, qubits or quantum dipole emitters resonantly coupled at optical frequencies with metal nanoparticles and one dimensional waveguides have been explored, opening the door to quantum plasmonics.6,7 Cavity quantum


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electrodynamics has been recently used for augmented emission by tuning into the different coupling regimes vis-à-vis weak, strong, ultra-strong and deep-strong coupling regimes.8,9 In addition, the strength of light-matter coupling can be quantified by vacuum Rabi frequency.10 The quantum Rabi model is written as: 1

𝐻𝑅𝑎𝑏𝑖 = 2 ħ𝜔𝑎 𝜎2 + ħ𝜔𝑟 𝑎† 𝑎 + ħ𝑔𝜎𝑥 (𝑎 + 𝑎 † ) ------- (1) Where a (a†) is the bosonic operator of the electromagnetic field mode with frequency ωr; ωa is the qubit transition frequency; σi (i = x, y, z) are the Pauli operators and g is dipole interaction strength. Depending on the value of g, one can define the coupling regimes which in turn can be tailored by modifying the light-matter interaction in a cavity.10 In the context of light-matter coupling, emission from a two-level emitter has been coupled to metallic thin film surface plasmons in Kretschmann and reverse Kretschmann configurations that are eventually out coupled into polarized, directional and enhanced emission.11 This technique called surface plasmon-coupled emission (SPCE) has recently been used to increase the collection efficiency of spontaneous emission from a radiating dipole and as a powerful tool for the investigation of protein monolayers12, observation of excited state moieties13, imaging of muscle14, and bio-sensing15, circumventing the limitations of conventional fluorescence spectroscopy. A traditional SPCE platform yields 3-7 fold enhancements in fluorescence.11 However, the enhancement factors could be further amplified by spacer engineering. Use of localized surface plasmons supported metal nanoparticles for achieving multifold enhancements of a dipole emitter was reported by Lakowicz et al.16 In our previous work, we demonstrated the use of spacer and cavity engineering to tune the spontaneous emission rate of a single dipole emitter and quantum dot based on simulations and experimentation to achieve femtomolar


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sensing of a dipole emitter17, picomolar level biosensing17 and evaluation of dye-DNA interaction.18 We also have exploited the spectral resolution properties of SPCE for operation in aqueous environement19, studying excited state molecules20, biomolecule sensing21 and understanding of interaction between organic dye and π conjugated carbon materials.17,22,23 Despite us achieving in excess of 1000 fold fluorescence enhancements from a radiating dipole, it was still restricted to the weak coupling regime.17 In the strong coupling regime, the strong exciton–cavity photon coupling leads to a quasi-lossless system with coherent oscillations of exciton and cavity photons.24 Here the electronic and optical states in the cavity merge to produce new states known as cavity-polaritons.25 So far, SPCE studies have not utilized cavity designing to stimulate the properties of strong coupling regime in fluorescence emission. In this work, we present the use of palladium and its composites as alternative plasmonic materials for fabrication of SPCE substrates for use in both the strong and weak coupling regimes and directed towards attomolar DNA detection and multi-analyte sensing using a smartphone. Results and Discussions: We adapt two configurations of the multi-layer stack namely ‘spacer’ and ‘cavity’ configurations by manipulating the sequence of the polymer layers (doped individually with the nanomaterial and the radiating dipole) spin coated on the SPCE substrate.23 The entrapment of the plasmonic nanomaterial doped polymer layer between the emitter polymer layer and the silver thin film yields the spacer configuration. However when the quantum emitter doped polymer is sandwiched between the nanomaterial doped polymer and the silver thin film, it is referred to as a cavity. The graphical representation of the multi-layer stack presenting the difference in the polymer free spacer and cavity configurations is shown in Figure 1a.


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Figure 1. (a) Graphical representation of spacer and cavity configurations, (b) optical scheme of SPCE platform, (c) tunable enhancements achieved with different spacer materials with standard deviations for triplicates. Spacer Engineering: In earlier reports, gold (AuNP) and silver nanoparticles (AgNP) alone were explored as spacers in SPCE substrates.16,26 We introduced the use of low-dimensional carbon nanospacers that


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resulted in ultra-amplification of enhancements in plasmon-coupled fluorescence emission intensities.17 However, there is no citation on the use of other noble metals as nanospacers in the SPCE platform. In this work, we demonstrate palladium based spacer engineering with the use of palladium nanoparticles (PdNP) and other palladium composites: 20 nm and 80 nm silver-core palladium-shell nanoparticles (Ag20@Pd, Ag80@Pd), gold-core palladium-shell nanoparticles (Au20@Pd), palladium-decorated graphene (Pd-G) and palladium-decorated multi-walled carbon nanotubes (Pd-CNT). Hemicylindrical prism, 532 nm green laser and detector: fiber coupled spectrometer or an android cellphone were mounted on a rotating stage to carry out SPCE studies (Figure 1b). Our studies revealed that PdNP spacer showed greater enhancement (̴60 fold) when compared with AgNP and AuNP spacers. The presence of palladium-shell on the AgNP core resulted in marginal enhancement compared to that of bare AgNP.26 However, the quenching action of bare AuNP on rhodamine 6G (Rh6G) emission was suppressed in the presence of palladium-shell, leading to enhancement in plasmon-coupled fluorescence intensity. It is further interesting to note that Ag20@Pd and Au20@Pd show similar enhancements irrespective of their different cores. However, with increased size of the core in Ag80@Pd, the enhancements dropped as expected.16,26 The contribution of the core to the fluorescence enhancements, seem to be negligible in comparison with the shell. This could possibly pave way to the utilization of non-plasmonic nanomaterials as the core for assorted applications. Figure 1c illustrates the plasmon-coupled emission enhancements obtained with different materials used as spacers on the SPCE substrate in the current study. Cavity engineering: In our earlier study, graphene and carbon nanotubes spacer layers have shown large enhancements in plasmon-coupled fluorescence intensity.17 However in comparison, PdCNT


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showed interesting properties. The scanning electron microscopy images of Pd-CNT have been presented in Figure 2a and 2b. Figure 2c depicts the plasmon-coupled fluorescence spectra in the case of undecorated graphene, CNT and bare PdNP. We observed that Pd-CNT as spacer showed remarkable spectral resolution (Figure 2d) similar to that of graphene. When these materials were used as cavity, we observed that the free space emission intensity reduces in line with the increasing plasmon-coupled emission intensity observed on the prism side. However, in the case of Pd-CNT, the measured free space emission was much lower than earlier observations (Figure 2e). Interestingly, the use of Pd-CNT as cavity material enabled us to tune into the strong coupling regime, validated by the observation of Rabi splitting (107.2 meV energy) of Rh6G emission at a 59° angle on the prism side (Figure 2f). This is the first observation of Rabi splitting in surface plasmon-coupled emission studies.


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Figure 2. (a,b) FESEM images of Pd-CNT, (c) SPCE of Rh6G obtained with blanks: PdNP, Graphene and CNT, (d) Rh6G emission maxima shift with Pd-CNT compared to Pd-G and PdNP, (e) free space Rh6G emission in cavity configuration for various materials and (f) prism side observation of Rabi splitting in Rh6G emission with reverse Kretschmann excitation.


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So far, the direct observation of Rabi splitting in the fluorescence emission of quantum emitters has been explored.27 Recently, it was shown that the surface plasmons also exhibit quantum phenomena like that of photons.28 In similar lines, in this work, the surface plasmons generated on the silver thin films, out couple on the prism side, resulting in Rabi splitting analogous to that of direct observation of fluorescence from an emitter in a strong coupling regime. This further validates the understanding that plasmons have the ability to carry quantum information like photons. Numerous systems such as micropillars, cavities have been used to study Rabi splitting of the spontaneous emission from an emitter that shows strong coupling with the cavity photons, entering the strong coupling regime.29,30 An earlier report has presented reflectivity based studies of Rabi Splitting in Rh6G emission at high concentration of the radiating dipole in the Kretschmann configuration.31,32 In the present study, Rabi splitting in Rh6G emission was observed for low concentration of the emitter, in reverse Kretschmann optical configuration. Purcell factor correlations: Having observed the exciting transition from the weak to the strong coupling regime, we further evaluated these regimes using Purcell factor (PF). We determined PF experimentally by carrying out time-correlated single photon counting (TCSPC) studies to evaluate the decay time of Rh6G in various cavity environments. Interestingly, the overall trend in fluorescence lifetimes associated with each of the cavities, correlated well with the corresponding plasmon-coupled fluorescence emission enhancements with a deviation only in the case of Pd-CNT (Figure 3a). This is suggestive that the emission in the Pd-CNT cavity environment is not irreversibly lost into the environment as a result of energy exchange, Rabi oscillations between emitter and the cavity field leading to Rabi splitting in the emission. This explains the unusually low free space emission in case of Pd-CNT spacer. Further, Casimir effect in both graphene and carbon


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nanotubes is well known.33,34 We believe that the extent of the Casimir interaction occurring in the case of Pd-G and Pd-CNT influences the shift in the coupling regimes. Further studies to understand this phenomenon is underway. The theoretical PF was calculated using finitedifference time-domain (FDTD) simulations for all the cavity materials and the effect of cavity size on PF was also determined (Figure 3b). We attribute the reduction in full width at half maxima (FWHM) of the plasmon-coupled emission with the use of PdNP, to the internal Fano resonance observed in the case of PdNP.35 The use of CNT as a quasi-1-dimensional cylindrical wire in enabling strong exiton-plasmon coupling has already been reported.36 In our opinion, the synergistic effect of both PdNP and CNT is seen in the Pd-CNT hybrid spacer, making it a tunable cavity for transition from weak to strong coupling regime in the SPCE platform. We believe that these exciting observations will further aid the exploration of other quantum plasmonic phenomena: quantum tunneling37, finesse38, Fowler-Nordheim tunneling39, Dicke effect40, Lamb shift41, whispering-gallery-mode resonators42, single-photon nonlinear optics43, using the SPCE platform.

Figure 3. (a) TCSPC plot showing fluorescence decay of Rh6G in graphene/Ag, CNT/Ag, PdNP/Ag and Pd-CNT/Ag nanocavities, (b) FDTD simulation study of the nanocavity size effect on Purcell Factor. Cellphone based sensing:


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We have carried forward the augmented SPCE enhancements achieved with Pd-CNT, towards DNA detection using a cellphone camera. With the advent of nanotechnology, cellphone camera based detection techniques have been gaining popularity.44 Here we show the use of cellphone camera as a detector for monitoring changes in Rh6G emission intensity at different DNA concentrations. Earlier reports account for the fluorescence quenching of Rh6G emission, based on its minor groove binding interaction with DNA.45-47 We exploit this understanding to arrive at a SPCE based technique for quantitative DNA detection with attomolar sensitivity. Higher concentrations of DNA result in greater quenching of Rh6G emission, accompanied with lesser plasmon-coupled fluorescence enhancements. To cross-validate the results and ensure consistency, we have carried out DNA detection studies with both the spectrometer assembly and the cellphone camera. The images of fluorescence emission from Rh6G were taken by aligning the cellphone camera with the emission angle. An in-built android camera app was used to take snapshots of the angular emission. We have used another open source android camera application ‘Camera FV-5 lite’, downloaded from Google Play store. This application enabled us to alter the exposure time and aperture size of the android camera. Further validation of the sensing capabilities of the cell phone camera, with nonspecific controls, involving the use of other leading mobile application platforms (iOS and Windows) was carried out to ensure compatibility, reproducibility and impact a broad audience. However since android is more popular than other smartphone platforms, we have used this platform for the entire study. The chromaticity was plotted with respect to the human eye color matching function defined by Commission Internationale de I’Eclairage (CIE).48 This widely used tristimulus values X, Y and Z are usually derived from the following equations:


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780

𝑋 = ∫ 𝐼 (𝜆)𝑥̅ (𝜆)𝑑𝜆 − − − −(2) 380 780

𝑌 = ∫ 𝐼 (𝜆)𝑦̅(𝜆)𝑑𝜆 − − − −(3) 380 780

𝑍 = ∫ 𝐼 (𝜆)𝑧̅(𝜆)𝑑𝜆 − − − −(4) 380

These trisimulus values are normalized to get

𝑥=

𝑋 − − − −(5) 𝑋+𝑌+𝑍

𝑦=

𝑌 − − − −(6) 𝑋+𝑌+𝑍

𝑧=

𝑍 − − − −(7) 𝑋+𝑌+𝑍

These obtained values are plotted in 1931 CIE chromaticity diagram. In this work, the images of angular emission were processed with an android application ‘Color Grab’ that exported the corresponding values of the concerned variables to RGB and CIE xyY color space. Figure 4a depicts a double Y-axis plot that presents the modification in the SPCE enhancements and R value (corresponding to red from RGB color space). We could detect DNA upto 1aM concentration, as further lowering of DNA concentration did not yield observable change in fluorescence emission intensity. We found that with and without long exposure settings, the cellphone camera could detect the fluorescence emission. However, when the emission was weak, the images were sharper in the case of long exposure settings. For future applications with cellphone camera based SPCE detection platform, we suggest the usage of long exposure settings to obtain micro-contrast, sharper pictures of weak emissions. It is important to mention here that the RGB color space does not represent all the visible colors and hence we have also processed


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the images in CIE xyY color space.48 Figure 4b is the chromaticity diagram that presents the plot of x versus y in the case of both, with and without long exposure settings. It is clear from figure 4b that in the case of normal settings, many data points overlap, resulting in an indiscernible image analysis. However, in the case of long exposure, each data point is clearly distinguishable and the variation in color can be clearly followed and analyzed. Figure 4c and 4d are images of Rh6G emission captured through the cellphone camera with and without long exposure settings. Figure 4e is the schematic representation of the substrate used for DNA detection.

Figure 4. (a) DNA detection with spectrometer, cellphone camera with and without long exposure settings (with standard deviations for triplicates), (b) chromaticity diagram depicting


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the change in fluorescence emission observed with cellphone camera, with and without long exposure setting (with standard deviations for triplicates). Cellphone camera emission images recorded with decreasing concentration of DNA (top to down): (c) normal settings, (d) long exposure settings, (e) schematic representation of the substrate used for DNA detection. Multi-analyte sensing is a very important capability needed for simultaneous evaluation of various parameters in bio-sensing applications. Given the innate ability of spectral resolution in SPCE, multi-analyte sensing without interference has been reported with the use of spectrometers. Here, we supplement the dual sensing capability using a cellphone camera. Lipoprotein-associated phospholipase A2 (LpPLA2) is a well-known coronary heart disease biomarker49 while fluorescein is extensively used in diagnostics, especially in the field of ophthalmology50. In this work, LpPLA2 antibody tagged with fluorescent quantum dot and fluorescein were admixed and coated on the plasmonic substrate with Pd-CNT for enhancing the emission. A graphical representation of the multi-layer stack used for multi-analyte detection, in cavity configuration is shown in Figure 5a. Both the emitters were taken at 1 picomolar concentration and illuminated with 405 nm c.w. laser. The emission from LpPLA2 labeled quantum dot is red while that of fluorescein is green. We have recorded the video of the spectrally resolved angular emission on the prism side, by placing the cellphone camera on a motorized stage. The video was processed to export frames at various points and the resultant image is plotted in the CIE space (Figure 5b). It is interesting to note the spectral resolution property represented in the CIE plot. Figure 5c depicts few frames obtained from the video presenting the distinct, multi-analyte sensing ability of the cellphone based SPCE platform.


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Figure 5. (a) Graphical representation of Pd-CNT on plasmonic silver thin film with quantum dot and fluorophore trapped in the cavity, (b) chromaticity diagram depicting the change in fluorescence emission observed with the cellphone camera for the multi-analyte system (with standard deviations for triplicates), (c) frames exported from the angular sweep video, depicting the capture of fluorescence emission from the two analytes.


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Conclusion: In summary, we have for the first time observed spacer layer dependent strong coupling regime dynamics in the SPCE platform, resulting in 168 fold enhancements in plasmon-coupled fluorescence emission. In addition to Pd-CNT and Pd-G composites, we have also studied the effect of Ag (or Au) core – Palladium shell structures on SPCE enhancements and have outlined its correlation with PF. Attomolar DNA detection and multi-analyte sensing were achieved with the use of cellphone camera based SPCE technique. We believe this study opens the door to innovative cellphone based point-of-care diagnostics and hand-held homeland security products based on the SPCE technique. We also envisage the exploration of quantum plasmonic effects with the use of this low-cost, home-built technology. Experimental Methods: All chemicals were purchased from Sigma-Aldrich. 50 nm silver thin films coated on pyrex slides with 5nm silica top layer were purchased from EMF Corp., USA. AgNP and AuNP were purchased from BBI international. Core-shell structures were prepared based on earlier reports.51,52 The multi-stack doped PVA layers containing 1 mg/ml of the nanomaterial and 1 mM Rh6G separately in 1% PVA were spin-coated at 3000 rpm for 60 seconds as reported earlier. 17,18,21,23,26 In case of multi-analyte detection, 1 pM concentration of both Rh6G and quantum dots were doped in 1% PVA and spin-coated at 3000 rpm for 60 seconds onto the SPCE substrate. Substrates were illuminated with a 532 nm c.w. laser (5 mW) and the emission was passed through 550 nm long-pass filter before collecting into the fiber coupled Ocean Optics 2000+ spectrometer. The angularity and enhancements were measured with the help of a rotating stage and a sheet polarizer was used for polarization measurements. Commercially available Lumerical FDTD Solutions was used for simulation studies. TCSPC studies were carried out


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using a 490nm LED pulsed laser and emission intensities were counted by Horiba Jobin Yvon TCSPC system and the fluorescence decay was analyzed using IBH- DAS V6.2. Commercially available Moto X running Android Lollipop 5.1 with 10 MP camera, Microsoft Lumia 950 with 20MP camera and iPhone 4S with 8MP camera were used for collection of emission images. Laser ablation mediated synthesis of Palladium composites- 1 mg of functionalized CNT was dispersed in 25 ml water to get a clear dispersion. A rectangular palladium strip (2 cm x 1 cm) was then placed in a beaker containing the aqueous dispersion of 3 ml CNT. A 1064 nm nanosecond laser beam from Nd:YAG source was optically steered and focused through a convex lens over the palladium strip. The focal length of 10 cm, pulse energy of 50 mJ and repetition rate of 10 Hz were kept constant throughout the 40 minute ablation of the strip resulting in the formation of PdNP and its decoration on CNT. Likewise, pristine PdNP (or PdG) were synthesized by ablation of Palladium strip in water (or a dispersion of graphene in water). The FESEM data was obtained from FESEM Zeiss instrument by drop casting few drops of the sample on an indium tin oxide ﬁlm. ASSOCIATED CONTENT Supporting Information Multimedia video capturing the multi-analyte sensing with the cellphone based SPCE platform. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected].


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT SSR and VS acknowledge the support from DBT-Ramalingaswamy fellowship (102/IFD/SAN/776/2015-16) and UGC-BSR fellowship, Govt. of India. Special thanks to Prof. P. Ramamurthy, National Centre for Ultrafast Processes, University of Madras and Department of Physics, Sri Sathya Sai Institute of Higher Learning for providing access to the time correlated single photon counting facility and laser ablation facility. We thank Sri. Sai Giridhar Sairam, SSSIHL for providing gratis sample of quantum dot labelled LpPLA2. Guidance from Bhagawan Sri Sathya Sai Baba is also gratefully acknowledged. ABBREVIATIONS PF, Purcell factor; SPCE, Surface plasmon-coupled emission; FDTD, Finite-difference timedomain; TCSPC, Time-correlated single-photon counting; Rh6G, Rhodamine 6G; Pd-G, Palladium loaded graphene; Pd-CNT, Palladium loaded carbon nanotube; PdNP, Palladium nanoparticles; Ag20@Pd, 20nm silver core with palladium shell; Ag80@Pd, 80nm silver core with palladium shell; Au20@Pd, 20nm gold core with palladium shell; AgNP, Silver nanoparticles; AuNP, Gold nanoparticles; CIE, Commission Internationale de I’Eclairage; LpPLA2, Lipoprotein-associated phospholipase A2.


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51) Chiu, C. Y.; Yang, M. Y.; Lin, F. C.; Huang, J. S.; Huang, M. H. Facile Synthesis of AuPd Core-Shell Nanocrystals with Systematic Shape Evolution and Tunable Size for Plasmonic Property Examination. Nanoscale. 2014, 6, 7656-7665. 52) Adekoya, J. A.; Dare, E. O.; Mesubi, M. A.; Nejo, A. A.; Swart, H. C.; Revaprasadu, N. Synthesis of Polyol Based Ag/Pd Nanocomposites for Applications. Results Phys. 2014, 4, 12-19.

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