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Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles
Journal Pre-proof Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles R. Fathima, A. Mujeeb PII:
S1386-1425(19)31103-5
DOI:
https://doi.org/10.1016/j.saa.2019.117713
Reference:
SAA 117713
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 28 August 2019 Revised Date:
25 October 2019
Accepted Date: 25 October 2019
Please cite this article as: R. Fathima, A. Mujeeb, Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117713. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles. Fathima R*, A Mujeeb International School of Photonics, CUSAT, Kochi-22, Kerala, India *E-mail: [email protected]
Abstract- Surfactant free gold, silver and gold-silver alloy nanoparticles were synthesized using a laser mediated method for localized surface plasmon resonance tuning. The effect of these nanoparticles on the thermo-optical properties of eosin B dye was investigated. Dual beam mode matched thermal lens method was implemented to evaluate the thermal diffusivity of the eosin B with gold, silver and gold-silver alloy nanoparticles. Concentration and composition dependant changes in thermo-optical properties of the eosin B-nanoparticle systems were quantified. As the concentration of nanoparticles incorporated into the dye solution increased, the thermal diffusivity and fluorescence emission intensity of the samples were found to be decreased. At the same time an enhancement of the thermal lens signal was observed with the introduction of nanoparticles into the system. Further enhancement in signal and reduction in thermal diffusivity and fluorescence intensity can be obtained with fine tuning of surface plasmon resonance wavelength by gold, silver and gold-silver alloy nanoparticles. Keywords- gold-silver alloy nanoparticles; Surface plasmon resonance; thermal diffusivity; thermal lens. 1.
INTRODUCTION
Plasmonic nanoparticles like gold and silver have gained much attention due to their wide applicability in sensing [1], catalysis [2], diagnosis, photo voltaics [3], photo thermal therapies [4], drug delivery etc. The unique optical properties of these nanoparticles arise from the collective oscillation of free electrons in the external electromagnetic field of incident light. This localized surface plasmon resonance (LSPR) leads to particular characteristics like intense absorption band in the visible region, local field enhancement [5, 6] etc. The peak extinction wavelength of localized surface plasmon resonance highly depends on the geometry and dielectric environment of the plasmonic nanoparticles. Transforming two or more plasmonic metals into a single nanoparticle can alter the plasmonic functionalities of the systems. The bimetallic nanoparticle structures like core-shell, hybrid or alloy structures exhibit superior and novel physical and chemical properties in selected conditions. On account of their exceptional stability, excellent bio compatibility and negligible toxicity, gold nanoparticles are the centre of attention in bio-medical applications. Silver nanoparticles possess superior plasmonic features like more sensitive LSPR peaks and stronger molar extinction coefficient, but the toxicity that originates from silver ion release in some biological systems limits their applicability in bio medical sectors. However,
alloying gold with silver nanoparticles results superior plasmonic features by suppressing the toxicity of silver ions [7, 8]. Conventional chemical reduction method itself induces toxicity in nanoparticles due to the presence of chemical stabilizers and ligands. In these processes, the presence of ligands on the nanoparticle surface has an additional impact on their properties and bio response. Hence laser ablation synthesis was selected as the synthesis method of the study to generate surfactant-free and impurity-free gold and silver nanoparticles [9]. Moreover, laser generated nanoparticles exhibits considerable colloidal stability attributable to the electrostatic interactions resulting from their negative surface charge. Also a laser irradiation induced method was used for the generation of colloidal gold-silver alloy nanoparticles from a colloidal mixture of gold and silver nanoparticles. Thermal lens (TL) technique is a highly sensitive photo thermal spectroscopic method adopted to investigate the thermo-optical properties and low absorption coefficient of comparatively transparent liquid samples. In TL method, the sample is excited by a Gaussian laser beam (pump beam) followed by the non-radiative relaxation. As a result, heat is generated in the sample in accordance with the laser intensity profile. Due to the temperature dependence of refractive index, the sample act like a lens. Another laser beam (probe beam) is used to monitor the thermal lens effect, which is diverged by the lens and as a result thermal blooming is generated [10, 11]. The thermal diffusivity studies are important in monitoring the nonradiative relaxation pathways of the excited dye molecules. It shed lights on many underlying processes related to light matter interactions and energy transfer pathways. The analysis of thermo-optical properties of the dye-nanoparticle system holds an important role in the process of designing energy efficient heat transfer systems. Moreover thermo-optic studies of plasmonic nanoparticles emerge to be preferably significant for applications in photo thermally activated drug delivery and in medical therapies [4]. Investigations on different dyes with metallic nano colloids are important in tuning the thermo-optical properties and fluorescence emission intensities [12-15]. Eosin-B dyes are bromine derivative of fluorescein which belongs to xanthene family of dyes (Figure 1). The optical properties of eosine B is a dynamic field of research owing to their application potentials as counterstain for visualizing proteins, connective tissues, cell granules, nuclei and microorganisms [16]. They are also employed as laser materials on account of high non linear optical susceptibility [17].
Figure 1 Molecular structure of eosin B (in double anion form).
In this article, the fine tuning of fluorescence emission intensities and thermo-optical properties of eosin B dye incorporating gold, silver and gold-silver alloy nanoparticles is reported. The plasmonic field of the nanoparticles alters the fluorescence emission of the dye located at its close proximity. The interaction between dye molecules and plasmonic nanoparticles can also alter the thermal diffusivity of the dye solution. The tailoring of the thermo-optical properties of the eosin B dye is advantageous in various applications like lasing which depends upon the photo thermal characteristics.
2.
EXPERIMENTAL
2.1. Synthesis of surfactant free gold and silver nanoparticles. The colloidal nano particles of silver and gold were separately synthesized by laser ablation of a silver and gold metal plate of 99.9% purity in deionized water without any surfactants or stabilizing agents. The ablation was performed with an Nd: YAG nano second laser (Spectra Physics Quanta Ray) with repetition rate of 10 Hz and pulse width of 10 ns. The targets were immersed in 8 ml deionized water taken in a 50 ml beaker. The pulsed laser beam of 1064 nm wavelength with pulse energy 50 mJ was focused by a biconvex lens of focal length 10 cm, onto the metal target. The time of laser ablation was fixed as 15 minutes by gently rotating the target manually.
2.2. Laser assisted synthesis of gold-silver alloy nanoparticles. The synthesized colloidal nanoparticles of gold and silver were mixed in 3 different volume ratios to get 12 ml solution (Table ST1:See Supplimentary data). The physical mixtures of individual colloidal solutions are
labeled as A, B and C which consists of gold and silver nano colloids in volume ratios (Au: Ag
1:2, 1:1,
and 2:1) respectively. The mixed dispersions were then re-irradiated with pulsed laser radiations from the same Nd: YAG nanosecond laser source at a wavelength 532 nm and 60 mJ pulse energy for 15 minutes. In order to ensure uniform irradiation, the sample was provided with horizontal and vertical translations. The alloy nanoparticles of different concentrations were then diluted with de ionized water in different volume ratios and labelled as samples a5 to a1, b5 to b1, and c5 to c1 (Table ST2-ST6:See supplementary data).
2.3. Addition of eosin B dye with gold, silver and gold-silver alloy nanoparticles. The eosin B dye stock solution was prepared at a concentration of 0.02 mM in deionized water. 3 ml of the dye solution is then mixed with 1 ml each of silver, gold and gold-silver alloy nanoparticle solutions and ultrasonicated for 15 minutes.
2.4. Thermal lens method The schematic diagram of the dual beam thermal lens set up employed to understand the changes in the thermo-optical properties of the dye-nano system is shown in Figure 2. In the present experimental set up, for the excitation of sample we use a continuous wave Diode Pumped Solid State laser (DPSS) with wavelength 403 nm (pump beam) and a 10 mW He-Ne laser at 632 nm as the probe beam to monitor the thermal lens formed. The probe and pump beams were aligned in a collinear configuration with the help of a dichroic mirror as shown. The pump beam is intensity modulated at a frequency of 3 Hz by means of a chopper. The collinear beams were then focused on to sample in a quartz cuvette of 10 mm path length by a lens of 15 cm focal length. The divergent probe beam was filtered and collected through a photo detector and digital storage oscilloscope.
Figure 2 Schematic diagram of the thermal lens setup.
To avoid the nonlinearity effects, the power of the pump beam was fixed to 10 mW. Thermo optical parameters of the sample can be retrieved from the thermal blooming. The time dependant decay of the probe beam was obtained as output thermal lens signal.
2.5. Instruments and characterization The optical properties of the samples were monitored using Jasco U-570 UV/ VIS/NIR spectrophotometer and the fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer with excitation wavelength 518 nm. The morphological and size studies were carried out using HRTEM images with 200 kV High Resolution TEM (Model: JEM2100). The fluorescence life time calculations were performed with the help of Delta Pro Fluorescence Life time system based on Time Correlated Single Photon Counting method (M/S Horiba Scientific). The size analyses of the nanoparticles were performed using dynamic light scattering (DLS) technique using HORIBA SZ-100 nanoparticle size and zeta potential analyzer.
3. RESULTS AND DISCUSSION When the colloidal mixtures of gold and silver nanoparticles were subjected to nanosecond laser irradiation at 532 nm for a particular period of time, a colour change was visually observed indicating
changes in the colloidal properties. The UV-Visible absorption spectra of the samples with time were obtained to monitor the changes in the plasmonic properties of the system. Figure 3 reports the UV-Visible spectra of the gold and silver nanoparticles respectively. The surface plasmon resonance peaks were obtained at 522 nm and 398 nm which confirmed the presence of colloidal gold and silver nanoparticles respectively [18]. The plasmon peak of silver was observed to be more symmetric and intensive in comparison with that of gold nanoparticles. This asymmetry and inferior plasmonic features of gold nanoparticle can be explained on the basis of plasmonic losses arising from the inter band transition [19, 20].
Figure 3 UV-Visible absorption spectra of gold (i) and silver (ii) nanoparticles.
The development of absorption spectrum of the colloidal mixtures of gold and silver nanoparticles in different volume ratios (sample A, B and C) with time is given in Figure 4.
Figure 4 Evolution of UV-Visible absorption spectra of samples A, B and C at different irradiation times.
In all the three samples, before irradiation the colloidal mixture showed two individual SPR peaks corresponds to the plasmonic resonance of gold and silver nanoparticles. The absorption spectra undergo drastic changes with time when the samples were exposed to laser irradiation. After 15 minutes of irradiation, the two observed individual SPR peaks were disappeared and a single peak was observed at the wavelength range which lies in between that of silver and gold nanoparticles. The single resonance peaks in the wavelength range intermediate to silver and gold nanoparticles are characteristic property of the goldsilver alloy nanoparticles [21-25]. The
photographs
of
gold,
silver
and
alloy
nanoparticle
samples
A,
B
and
C is given in Figure 5. It has been demonstrated in literature that it is possible to heat the nanoparticles above their melting point through localized heating process if the nanoparticles possess sufficient
absorbance at the particular wavelength of laser irradiation [26]. As the gold and silver nanoparticles in the samples have considerable absorbance in the laser wavelength of 532 nm, the laser irradiation leads to laser induced melting and inter diffusion of nanoparticles, which in turn paves way to the alloy nanoparticle formation [27]. The LSPR peak of gold nanoparticles exhibits a more significant blue shift on the first minutes of laser irradiation of the colloidal mixtures. Even though the intensity of the absorption peak corresponding to segregated silver nanoparticles is considerably reduced with laser irradiation, the spectral shift is minimal in comparison with gold nanoparticles. The variations and shift in absorption spectra of the colloidal mixture can be explained on the basis of inter diffusion of silver nanoparticles into gold nanoparticles. Owing to the greater mobility of smaller silver atoms, similarity in lattice parameters and space groups their interdiffusion to gold nanoparticles is feasible which explains LSPR shifts and reduction in intensity [28, 29]. With further laser irradiation, the LSPR peaks merge into a single peak which indicates the formation of gold-silver alloy nanoparticles. When gold-silver alloy nanoparticles are formed as a result of laser irradiation, the d band energy levels suffer perturbations. The variations of interband transition energy threshold due to alloying resulted in a broad single LSPR peak [28, 30]. The SPR peaks of the gold-silver alloy nanoparticles from sample A, B and C were obtained as 404 nm, 452 nm and 480 nm. The entire plasmon band was subjected to a red shift as the volume percentage of gold nanoparticles in the alloy samples increases. This property can be used to have a control over the plasmonic resonance wavelength which is advantageous in various applications like fluorescence tuning, Surface Enhanced Raman Spectroscopy etc.
Figure 5 Photograph of silver (S), gold (G), and gold-silver alloy nanoparticles (A, B, and C).
The single SPR peaks obtained in the UV-Visible absorption spectra of the synthesized nanoparticle samples imply nearly spherical structure of the nanoparticles. The HRTEM image and corresponding size distribution of the gold-silver alloy nanoparticle for the sample B is shown in Figure 6. The spherical structure of the alloy nanoparticles is confirmed from the HRTEM image. The HRTEM micrographs exhibit the crystalline nature of the nanoparticle that attributes to the presence of lattice fringes with a spacing of nearly 0.23 nm. The lattice plane spacing obtained here is compatible with pure Ag (0.236) and Au (0.235 nm) which suggest the alloy nanoparticle formation [31]. The average size of the nanoparticles was found to be 24.5 nm from the analysis of HRTEM micrographs.
Figure 6 HRTEM images of the gold-silver alloy nanoparticles (sample B).
The size analysis of the nanoparticles was performed using dynamic light scattering (DLS) technique. The average diameters of the nanoparticles obtained were reported in Table 1.
Table 1.Size of the nanoparticles determined from DLS. Sample
Average diameter
Standard deviation
name
(nm)
(nm)
S
21.1
7.2
A
25.7
6.8
B
27.3
7
C
30.5
7.9
G
29.6
7
The optical absorption spectra of eosin B dye and dye-nanoparticle system is shown in figure SF1 (See Supplementary data). In aqueous medium the dye exhibits an intense absorption peak at 518 nm. As the concentration of alloy nanoparticles in the dye-nanoparticle colloidal system was increased, the absorption peak intensity was subjected to slight enhancement due to the presence of plasmonic field of gold-silver alloy nanoparticles. A broadening of the absorption band can also observed in the visible region of the spectrum. Photo excitation of eosin B at 518 nm yields an intense fluorescence emission around 541 nm. The fluorescence emission spectra of eosin B dye and the eosin B-gold silver alloy nanoparticles systems are given in figure 7. Successive addition of increasing concentrations of gold-silver alloy nanoparticles resulted in a prominent quenching of the fluorescence emission intensity.
Figure 7 Fluorescence emission spectra of eosin B dye (i) and eosin B-gold silver alloy nanoparticle samples A, B and C (ii, iii and iv).
It was observed that the quenching was more remarkable in the case of Au-rich gold-silver alloy nanoparticles samples B and C in comparison with Ag-rich sample A. The fluorescence quenching efficiency exhibits a tendency of enhancement with the volume fraction of gold nanoparticles in the colloidal nano samples (Figure 8). It can be explained on the basis of increased overlap between the emission wavelength of the eosin B dye and absorption spectra of nano particles which could possibly lead to a more eminent energy transfer from dye to plasmonic nanoparticles.
Figure 8 Fluorescence emission intensities of eosin B dye with nanoparticle samples S, A, B, C and G.
Fluorescence lifetime measurements were carried out employing Time-correlated single photon counting (TCSPC) method. The fluorescence intensities decays with time according to the expression Where t is the time after the absorption, F0 is the fluorescence intensity at t=0 and τ is the fluorescence life time of the sample. From the slope of the decay curve according to the above equation, the fluorescence life time of the sample can be calculated. If the decay is multi exponential, the corresponding equation is given as
Least square curve fitting algorithm was employed to extract the fluorescence life time values [32]. The fluorescence life times of the eosin B dye with gold, silver and alloy nanoparticle samples are reported in Table 2.
Table 2.Fluorescence life times of the samples Sample name
Average Lifetime (ns)
D (eosin B dye )
1.191
S
1.145
A
1.140
B
1.139
C
1.138
G
1.132
With the introduction of nanoparticles into the system, the life time values were found to be slightly decreased which points towards the possibility of energy transfer between dye molecules and nanoparticles. The reduction in the fluorescence life time in the presence of plasmonic nanoparticles suggests the dynamic nature of quenching [33]. Due to the increased number of the acceptor species (plasmonic nanoparticles), the energy transfer also increased with concentration of nanoparticle, which explains the improved quenching of fluorescence. To get an insight to the photo thermal properties and heat transfer mechanism in the eosin B dye-gold silver alloy nanoparticle samples, thermal lens method was adopted [34, 35]. Considering thermal diffusion as the main heat transfer mechanism, the variation in thermal diffusivity with the concentration of gold, silver and gold silver alloy nanoparticles in the eosin B- nanoparticle system were studied.
Figure 9 reports the
typical thermal lens signal obtained for eosin B dye sample at 3Hz pump intensity modulation.
Figure 9 Typical Thermal Lens signal obtained from eosin B dye sample solution.
Probe beam intensity decays with time according to the expression given below. [1 –
1 +
+
1 +
]
Where tc is the characteristic time for diffusion. The parameter θ is defined as
[
[!" #]
Where Pth denotes the thermal power dispersed as heat and
] $% $&
is the temperature co-efficient of refractive
index, !" is the wavelength of probe laser and k is the thermal conductivity of the sample. The characteristic time for diffusion tc and θ can be determined by curve fitting of experimentally obtained probe beam intensity decay values to equation for I(t). The thermal diffusivity D was calculated from the tc values using the below relation. '
( / 4+
The normalized intensity decay of the probe beam with time and the corresponding theoretical fitted curves of eosin B dye samples with different concentration of gold-silver alloy nanoparticles of sample A, B and C is reported as Figure 10. An enhancement in the thermal lens signal amplitude can be observed as the concentration of gold-silver alloy nanoparticles introduced into the system increases. This signal enhancements offer an additional advantage of easy detection of samples in thermal lens spectroscopy.
Figure 10 The normalized intensity decay of the probe beam with time of eosin B dye samples with different concentration of gold-silver alloy nanoparticles of sample A (i), B(ii) and C(iii).
The thermal lens signal emanates from nonradiative relaxation of excited particles when they are subjected to laser irradiation. These nonradiative relaxation processes consist of vibrational relaxation, external conversion and intersystem crossing. As a result of these processes the energy absorbed from laser irradiation is converted into the form of heat. The amount of heat generated in the sample solution is the key factor determining the magnitude of the thermal lens signal. The governing factors of heat generation in the samples are the excitation laser power, absorbance of the samples etc. The fluorescence efficiency of the samples is also a crucial property which negatively influences the heat generation. The signal magnitude enhancement is also influenced by the thermo-optical properties of the sample material like thermal diffusivity D and temperature co-efficient of refractive index [36]. The thermal diffusivity values and fluorescence emission intensities of the eosin B samples with increasing concentrations of gold-silver alloy nanoparticles of sample A, B and C are reported in Table
ST7, ST8 and ST9 respectively (See
Supplementary data).
It is evident from the Table S7, S8 and S9 that the thermal diffusivity and fluorescence emission intensity values were decreased with increase in concentration of nanoparticles incorporated into the eosin B dye where as the thermal lens signal amplitude was found to be enhanced. The reduction in fluorescence emission intensity in turn contributed to the enhancement of TL signal.
As the power of the pump beam was kept constant throughout the experiment, the determining features that contributed to the enhancement of thermal lens signal was the improvement in the ratio of non-radiate to radiative emission of the dye-nanoparticle system due to fluorescence quenching of the dye in the presence of nanoparticles and the broadening of absorption spectrum of the dye with introduction of nanoparticles [36]. The reduction in the thermal diffusivity of the dye with addition of plasmonic nanoparticles can be explained on the basis of cluster formation between nanoparticles and dye molecules [37, 34]. Even though there is possibility of effective heat transfer within the cluster, the heat exchange with the base fluid is negatively affected. The heat diffusion between dye molecules and base fluid turned slower due to this clustering effect. A Comparison of the effect of silver, gold and gold –silver alloy nanoparticles (samples G, S and A, B, C) on the thermal lens signal decay of the eosin B dye is illustrated in Figure 11. Corresponding values of thermo-optical parameters like thermal diffusivity D, characteristic time constant tc and fluorescence emission intensity is given as table 3.
Table 3 Thermo-optical parameters of eosin B with silver, gold and gold-silver alloy nanoparticles.
Sample Name
Time constant
Thermal
Thermal lens
Fluorescence
tc (s)
diffusivity
signal amplitude
emission
-8
2
D (10 m /s)
intensity
D
0.0584
6.257
0.04132
987.43
D+S
0.1914
1.909
0.11628
808.73
D+A
0.1300
2.811
0.10484
798.66
D+B
0.1120
3.262
0.09067
721.15
D+C
0.1074
3.402
0.09434
739.15
D+G
0.0923
3.958
0.08333
656.79
Figure 11 (a) The normalized intensity decay of the probe beam with time of eosin B dye samples with gold, silver and gold-silver alloy nanoparticles. (b) Amplitude of the thermal lens signal and thermal diffusivity of eosin B dye samples with gold, silver and gold-silver alloy nanoparticles.
It was observed that the amplitude of thermal lens signal was further enhanced by changing the composition of the nanoparticle samples incorporated into the system. With the blue shift in SPR peak from samples G to S, the thermal lens signal amplitude increased. The major factor contributing to the enhancement is the broadening of absorption spectrum of dye with the introduction of nanoparticle and the fluorescence quenching by nanoparticles. The absorbance at the pump beam wavelength was considerably improved by SPR tuning which in turn resulted in amplified thermal lens signal and faster thermal decay.
The thermal diffusivity was found to be highest for dye alone samples. With the introduction of plasmonic nanoparticles into the system, an improvement in the decay rate or reduction in thermal diffusivity was
observed which depends on the fluorescence quenching efficiency and absorbance enhancement. This leads to the possibility of fine tuning of the thermo optical properties of eosin B dye with nanoparticles. Further reduction in thermal diffusivity can be achieved with the surface plasmon tuning towards pump beam wavelength using silver-gold alloy nanoparticles.
4. CONCULSION
The influence of plasmonic nanoparticles on the thermo optical properties of eosin B dye was investigated. With the introduction of laser ablated gold, silver and gold-silver alloy nanoparticles to the eosin B dye solution, the thermal diffusivity and fluorescence emission intensity of the samples were decreased where as the thermal lens signal was found to be enhanced. The reduction of thermal diffusivity and amplification of TL signal was observed to be enlarged with the concentration of nanoparticles incorporated into the dye solution. The improvement in the ratio of non-radiate to radiative emission of the dye-nanoparticle system due to fluorescence quenching of the dye in the presence of nanoparticles and the broadening of absorption spectrum of the dye with introduction of nanoparticles have major role in the enhancement of thermal lens signal. The variation of thermal diffusivity, thermal lens signal amplitude and fluorescence emission intensity with surface plasmon peak tuning was analyzed using gold-silver alloy nanoparticles. It was found that further reduction of thermal diffusivity and enhancement of TL signal is possible by tuning SPR band with gold-silver alloy nanoparticles. Hence the present study concludes that fine tuning of thermo optical properties of eosin B dye can be made possible by varying the nanoparticle concentration and composition. The investigations on thermo-optical properties of eosin B incorporated with plasmonic nanoparticles have significance in photothermal applications of the dye like lasing, laser induced fluorescence studies, photo thermal imaging and improving the sensitivity of the fluorescence sensing systems.
Acknowledgement
The first author acknowledges the financial support from CSIR for research funding.
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Highlights: • •
•
Thermo-optical properties of Eosin B dye can be tuned with laser synthesized gold-silver alloy nanoparticles. A reduction in thermal diffusivity and enhancement in thermal lens signal of eosin B as with incorporation of gold-silver alloy nanoparticles. Effective tuning by changing composition and concentration of nanoparticles.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S1386-1425(19)31103-5
DOI:
https://doi.org/10.1016/j.saa.2019.117713
Reference:
SAA 117713
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 28 August 2019 Revised Date:
25 October 2019
Accepted Date: 25 October 2019
Please cite this article as: R. Fathima, A. Mujeeb, Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117713. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Tailoring thermo-optical properties of eosin B dye using surfactant-free gold-silver alloy nanoparticles. Fathima R*, A Mujeeb International School of Photonics, CUSAT, Kochi-22, Kerala, India *E-mail: [email protected]
Abstract- Surfactant free gold, silver and gold-silver alloy nanoparticles were synthesized using a laser mediated method for localized surface plasmon resonance tuning. The effect of these nanoparticles on the thermo-optical properties of eosin B dye was investigated. Dual beam mode matched thermal lens method was implemented to evaluate the thermal diffusivity of the eosin B with gold, silver and gold-silver alloy nanoparticles. Concentration and composition dependant changes in thermo-optical properties of the eosin B-nanoparticle systems were quantified. As the concentration of nanoparticles incorporated into the dye solution increased, the thermal diffusivity and fluorescence emission intensity of the samples were found to be decreased. At the same time an enhancement of the thermal lens signal was observed with the introduction of nanoparticles into the system. Further enhancement in signal and reduction in thermal diffusivity and fluorescence intensity can be obtained with fine tuning of surface plasmon resonance wavelength by gold, silver and gold-silver alloy nanoparticles. Keywords- gold-silver alloy nanoparticles; Surface plasmon resonance; thermal diffusivity; thermal lens. 1.
INTRODUCTION
Plasmonic nanoparticles like gold and silver have gained much attention due to their wide applicability in sensing [1], catalysis [2], diagnosis, photo voltaics [3], photo thermal therapies [4], drug delivery etc. The unique optical properties of these nanoparticles arise from the collective oscillation of free electrons in the external electromagnetic field of incident light. This localized surface plasmon resonance (LSPR) leads to particular characteristics like intense absorption band in the visible region, local field enhancement [5, 6] etc. The peak extinction wavelength of localized surface plasmon resonance highly depends on the geometry and dielectric environment of the plasmonic nanoparticles. Transforming two or more plasmonic metals into a single nanoparticle can alter the plasmonic functionalities of the systems. The bimetallic nanoparticle structures like core-shell, hybrid or alloy structures exhibit superior and novel physical and chemical properties in selected conditions. On account of their exceptional stability, excellent bio compatibility and negligible toxicity, gold nanoparticles are the centre of attention in bio-medical applications. Silver nanoparticles possess superior plasmonic features like more sensitive LSPR peaks and stronger molar extinction coefficient, but the toxicity that originates from silver ion release in some biological systems limits their applicability in bio medical sectors. However,
alloying gold with silver nanoparticles results superior plasmonic features by suppressing the toxicity of silver ions [7, 8]. Conventional chemical reduction method itself induces toxicity in nanoparticles due to the presence of chemical stabilizers and ligands. In these processes, the presence of ligands on the nanoparticle surface has an additional impact on their properties and bio response. Hence laser ablation synthesis was selected as the synthesis method of the study to generate surfactant-free and impurity-free gold and silver nanoparticles [9]. Moreover, laser generated nanoparticles exhibits considerable colloidal stability attributable to the electrostatic interactions resulting from their negative surface charge. Also a laser irradiation induced method was used for the generation of colloidal gold-silver alloy nanoparticles from a colloidal mixture of gold and silver nanoparticles. Thermal lens (TL) technique is a highly sensitive photo thermal spectroscopic method adopted to investigate the thermo-optical properties and low absorption coefficient of comparatively transparent liquid samples. In TL method, the sample is excited by a Gaussian laser beam (pump beam) followed by the non-radiative relaxation. As a result, heat is generated in the sample in accordance with the laser intensity profile. Due to the temperature dependence of refractive index, the sample act like a lens. Another laser beam (probe beam) is used to monitor the thermal lens effect, which is diverged by the lens and as a result thermal blooming is generated [10, 11]. The thermal diffusivity studies are important in monitoring the nonradiative relaxation pathways of the excited dye molecules. It shed lights on many underlying processes related to light matter interactions and energy transfer pathways. The analysis of thermo-optical properties of the dye-nanoparticle system holds an important role in the process of designing energy efficient heat transfer systems. Moreover thermo-optic studies of plasmonic nanoparticles emerge to be preferably significant for applications in photo thermally activated drug delivery and in medical therapies [4]. Investigations on different dyes with metallic nano colloids are important in tuning the thermo-optical properties and fluorescence emission intensities [12-15]. Eosin-B dyes are bromine derivative of fluorescein which belongs to xanthene family of dyes (Figure 1). The optical properties of eosine B is a dynamic field of research owing to their application potentials as counterstain for visualizing proteins, connective tissues, cell granules, nuclei and microorganisms [16]. They are also employed as laser materials on account of high non linear optical susceptibility [17].
Figure 1 Molecular structure of eosin B (in double anion form).
In this article, the fine tuning of fluorescence emission intensities and thermo-optical properties of eosin B dye incorporating gold, silver and gold-silver alloy nanoparticles is reported. The plasmonic field of the nanoparticles alters the fluorescence emission of the dye located at its close proximity. The interaction between dye molecules and plasmonic nanoparticles can also alter the thermal diffusivity of the dye solution. The tailoring of the thermo-optical properties of the eosin B dye is advantageous in various applications like lasing which depends upon the photo thermal characteristics.
2.
EXPERIMENTAL
2.1. Synthesis of surfactant free gold and silver nanoparticles. The colloidal nano particles of silver and gold were separately synthesized by laser ablation of a silver and gold metal plate of 99.9% purity in deionized water without any surfactants or stabilizing agents. The ablation was performed with an Nd: YAG nano second laser (Spectra Physics Quanta Ray) with repetition rate of 10 Hz and pulse width of 10 ns. The targets were immersed in 8 ml deionized water taken in a 50 ml beaker. The pulsed laser beam of 1064 nm wavelength with pulse energy 50 mJ was focused by a biconvex lens of focal length 10 cm, onto the metal target. The time of laser ablation was fixed as 15 minutes by gently rotating the target manually.
2.2. Laser assisted synthesis of gold-silver alloy nanoparticles. The synthesized colloidal nanoparticles of gold and silver were mixed in 3 different volume ratios to get 12 ml solution (Table ST1:See Supplimentary data). The physical mixtures of individual colloidal solutions are
labeled as A, B and C which consists of gold and silver nano colloids in volume ratios (Au: Ag
1:2, 1:1,
and 2:1) respectively. The mixed dispersions were then re-irradiated with pulsed laser radiations from the same Nd: YAG nanosecond laser source at a wavelength 532 nm and 60 mJ pulse energy for 15 minutes. In order to ensure uniform irradiation, the sample was provided with horizontal and vertical translations. The alloy nanoparticles of different concentrations were then diluted with de ionized water in different volume ratios and labelled as samples a5 to a1, b5 to b1, and c5 to c1 (Table ST2-ST6:See supplementary data).
2.3. Addition of eosin B dye with gold, silver and gold-silver alloy nanoparticles. The eosin B dye stock solution was prepared at a concentration of 0.02 mM in deionized water. 3 ml of the dye solution is then mixed with 1 ml each of silver, gold and gold-silver alloy nanoparticle solutions and ultrasonicated for 15 minutes.
2.4. Thermal lens method The schematic diagram of the dual beam thermal lens set up employed to understand the changes in the thermo-optical properties of the dye-nano system is shown in Figure 2. In the present experimental set up, for the excitation of sample we use a continuous wave Diode Pumped Solid State laser (DPSS) with wavelength 403 nm (pump beam) and a 10 mW He-Ne laser at 632 nm as the probe beam to monitor the thermal lens formed. The probe and pump beams were aligned in a collinear configuration with the help of a dichroic mirror as shown. The pump beam is intensity modulated at a frequency of 3 Hz by means of a chopper. The collinear beams were then focused on to sample in a quartz cuvette of 10 mm path length by a lens of 15 cm focal length. The divergent probe beam was filtered and collected through a photo detector and digital storage oscilloscope.
Figure 2 Schematic diagram of the thermal lens setup.
To avoid the nonlinearity effects, the power of the pump beam was fixed to 10 mW. Thermo optical parameters of the sample can be retrieved from the thermal blooming. The time dependant decay of the probe beam was obtained as output thermal lens signal.
2.5. Instruments and characterization The optical properties of the samples were monitored using Jasco U-570 UV/ VIS/NIR spectrophotometer and the fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer with excitation wavelength 518 nm. The morphological and size studies were carried out using HRTEM images with 200 kV High Resolution TEM (Model: JEM2100). The fluorescence life time calculations were performed with the help of Delta Pro Fluorescence Life time system based on Time Correlated Single Photon Counting method (M/S Horiba Scientific). The size analyses of the nanoparticles were performed using dynamic light scattering (DLS) technique using HORIBA SZ-100 nanoparticle size and zeta potential analyzer.
3. RESULTS AND DISCUSSION When the colloidal mixtures of gold and silver nanoparticles were subjected to nanosecond laser irradiation at 532 nm for a particular period of time, a colour change was visually observed indicating
changes in the colloidal properties. The UV-Visible absorption spectra of the samples with time were obtained to monitor the changes in the plasmonic properties of the system. Figure 3 reports the UV-Visible spectra of the gold and silver nanoparticles respectively. The surface plasmon resonance peaks were obtained at 522 nm and 398 nm which confirmed the presence of colloidal gold and silver nanoparticles respectively [18]. The plasmon peak of silver was observed to be more symmetric and intensive in comparison with that of gold nanoparticles. This asymmetry and inferior plasmonic features of gold nanoparticle can be explained on the basis of plasmonic losses arising from the inter band transition [19, 20].
Figure 3 UV-Visible absorption spectra of gold (i) and silver (ii) nanoparticles.
The development of absorption spectrum of the colloidal mixtures of gold and silver nanoparticles in different volume ratios (sample A, B and C) with time is given in Figure 4.
Figure 4 Evolution of UV-Visible absorption spectra of samples A, B and C at different irradiation times.
In all the three samples, before irradiation the colloidal mixture showed two individual SPR peaks corresponds to the plasmonic resonance of gold and silver nanoparticles. The absorption spectra undergo drastic changes with time when the samples were exposed to laser irradiation. After 15 minutes of irradiation, the two observed individual SPR peaks were disappeared and a single peak was observed at the wavelength range which lies in between that of silver and gold nanoparticles. The single resonance peaks in the wavelength range intermediate to silver and gold nanoparticles are characteristic property of the goldsilver alloy nanoparticles [21-25]. The
photographs
of
gold,
silver
and
alloy
nanoparticle
samples
A,
B
and
C is given in Figure 5. It has been demonstrated in literature that it is possible to heat the nanoparticles above their melting point through localized heating process if the nanoparticles possess sufficient
absorbance at the particular wavelength of laser irradiation [26]. As the gold and silver nanoparticles in the samples have considerable absorbance in the laser wavelength of 532 nm, the laser irradiation leads to laser induced melting and inter diffusion of nanoparticles, which in turn paves way to the alloy nanoparticle formation [27]. The LSPR peak of gold nanoparticles exhibits a more significant blue shift on the first minutes of laser irradiation of the colloidal mixtures. Even though the intensity of the absorption peak corresponding to segregated silver nanoparticles is considerably reduced with laser irradiation, the spectral shift is minimal in comparison with gold nanoparticles. The variations and shift in absorption spectra of the colloidal mixture can be explained on the basis of inter diffusion of silver nanoparticles into gold nanoparticles. Owing to the greater mobility of smaller silver atoms, similarity in lattice parameters and space groups their interdiffusion to gold nanoparticles is feasible which explains LSPR shifts and reduction in intensity [28, 29]. With further laser irradiation, the LSPR peaks merge into a single peak which indicates the formation of gold-silver alloy nanoparticles. When gold-silver alloy nanoparticles are formed as a result of laser irradiation, the d band energy levels suffer perturbations. The variations of interband transition energy threshold due to alloying resulted in a broad single LSPR peak [28, 30]. The SPR peaks of the gold-silver alloy nanoparticles from sample A, B and C were obtained as 404 nm, 452 nm and 480 nm. The entire plasmon band was subjected to a red shift as the volume percentage of gold nanoparticles in the alloy samples increases. This property can be used to have a control over the plasmonic resonance wavelength which is advantageous in various applications like fluorescence tuning, Surface Enhanced Raman Spectroscopy etc.
Figure 5 Photograph of silver (S), gold (G), and gold-silver alloy nanoparticles (A, B, and C).
The single SPR peaks obtained in the UV-Visible absorption spectra of the synthesized nanoparticle samples imply nearly spherical structure of the nanoparticles. The HRTEM image and corresponding size distribution of the gold-silver alloy nanoparticle for the sample B is shown in Figure 6. The spherical structure of the alloy nanoparticles is confirmed from the HRTEM image. The HRTEM micrographs exhibit the crystalline nature of the nanoparticle that attributes to the presence of lattice fringes with a spacing of nearly 0.23 nm. The lattice plane spacing obtained here is compatible with pure Ag (0.236) and Au (0.235 nm) which suggest the alloy nanoparticle formation [31]. The average size of the nanoparticles was found to be 24.5 nm from the analysis of HRTEM micrographs.
Figure 6 HRTEM images of the gold-silver alloy nanoparticles (sample B).
The size analysis of the nanoparticles was performed using dynamic light scattering (DLS) technique. The average diameters of the nanoparticles obtained were reported in Table 1.
Table 1.Size of the nanoparticles determined from DLS. Sample
Average diameter
Standard deviation
name
(nm)
(nm)
S
21.1
7.2
A
25.7
6.8
B
27.3
7
C
30.5
7.9
G
29.6
7
The optical absorption spectra of eosin B dye and dye-nanoparticle system is shown in figure SF1 (See Supplementary data). In aqueous medium the dye exhibits an intense absorption peak at 518 nm. As the concentration of alloy nanoparticles in the dye-nanoparticle colloidal system was increased, the absorption peak intensity was subjected to slight enhancement due to the presence of plasmonic field of gold-silver alloy nanoparticles. A broadening of the absorption band can also observed in the visible region of the spectrum. Photo excitation of eosin B at 518 nm yields an intense fluorescence emission around 541 nm. The fluorescence emission spectra of eosin B dye and the eosin B-gold silver alloy nanoparticles systems are given in figure 7. Successive addition of increasing concentrations of gold-silver alloy nanoparticles resulted in a prominent quenching of the fluorescence emission intensity.
Figure 7 Fluorescence emission spectra of eosin B dye (i) and eosin B-gold silver alloy nanoparticle samples A, B and C (ii, iii and iv).
It was observed that the quenching was more remarkable in the case of Au-rich gold-silver alloy nanoparticles samples B and C in comparison with Ag-rich sample A. The fluorescence quenching efficiency exhibits a tendency of enhancement with the volume fraction of gold nanoparticles in the colloidal nano samples (Figure 8). It can be explained on the basis of increased overlap between the emission wavelength of the eosin B dye and absorption spectra of nano particles which could possibly lead to a more eminent energy transfer from dye to plasmonic nanoparticles.
Figure 8 Fluorescence emission intensities of eosin B dye with nanoparticle samples S, A, B, C and G.
Fluorescence lifetime measurements were carried out employing Time-correlated single photon counting (TCSPC) method. The fluorescence intensities decays with time according to the expression Where t is the time after the absorption, F0 is the fluorescence intensity at t=0 and τ is the fluorescence life time of the sample. From the slope of the decay curve according to the above equation, the fluorescence life time of the sample can be calculated. If the decay is multi exponential, the corresponding equation is given as
Least square curve fitting algorithm was employed to extract the fluorescence life time values [32]. The fluorescence life times of the eosin B dye with gold, silver and alloy nanoparticle samples are reported in Table 2.
Table 2.Fluorescence life times of the samples Sample name
Average Lifetime (ns)
D (eosin B dye )
1.191
S
1.145
A
1.140
B
1.139
C
1.138
G
1.132
With the introduction of nanoparticles into the system, the life time values were found to be slightly decreased which points towards the possibility of energy transfer between dye molecules and nanoparticles. The reduction in the fluorescence life time in the presence of plasmonic nanoparticles suggests the dynamic nature of quenching [33]. Due to the increased number of the acceptor species (plasmonic nanoparticles), the energy transfer also increased with concentration of nanoparticle, which explains the improved quenching of fluorescence. To get an insight to the photo thermal properties and heat transfer mechanism in the eosin B dye-gold silver alloy nanoparticle samples, thermal lens method was adopted [34, 35]. Considering thermal diffusion as the main heat transfer mechanism, the variation in thermal diffusivity with the concentration of gold, silver and gold silver alloy nanoparticles in the eosin B- nanoparticle system were studied.
Figure 9 reports the
typical thermal lens signal obtained for eosin B dye sample at 3Hz pump intensity modulation.
Figure 9 Typical Thermal Lens signal obtained from eosin B dye sample solution.
Probe beam intensity decays with time according to the expression given below. [1 –
1 +
+
1 +
]
Where tc is the characteristic time for diffusion. The parameter θ is defined as
[
[!" #]
Where Pth denotes the thermal power dispersed as heat and
] $% $&
is the temperature co-efficient of refractive
index, !" is the wavelength of probe laser and k is the thermal conductivity of the sample. The characteristic time for diffusion tc and θ can be determined by curve fitting of experimentally obtained probe beam intensity decay values to equation for I(t). The thermal diffusivity D was calculated from the tc values using the below relation. '
( / 4+
The normalized intensity decay of the probe beam with time and the corresponding theoretical fitted curves of eosin B dye samples with different concentration of gold-silver alloy nanoparticles of sample A, B and C is reported as Figure 10. An enhancement in the thermal lens signal amplitude can be observed as the concentration of gold-silver alloy nanoparticles introduced into the system increases. This signal enhancements offer an additional advantage of easy detection of samples in thermal lens spectroscopy.
Figure 10 The normalized intensity decay of the probe beam with time of eosin B dye samples with different concentration of gold-silver alloy nanoparticles of sample A (i), B(ii) and C(iii).
The thermal lens signal emanates from nonradiative relaxation of excited particles when they are subjected to laser irradiation. These nonradiative relaxation processes consist of vibrational relaxation, external conversion and intersystem crossing. As a result of these processes the energy absorbed from laser irradiation is converted into the form of heat. The amount of heat generated in the sample solution is the key factor determining the magnitude of the thermal lens signal. The governing factors of heat generation in the samples are the excitation laser power, absorbance of the samples etc. The fluorescence efficiency of the samples is also a crucial property which negatively influences the heat generation. The signal magnitude enhancement is also influenced by the thermo-optical properties of the sample material like thermal diffusivity D and temperature co-efficient of refractive index [36]. The thermal diffusivity values and fluorescence emission intensities of the eosin B samples with increasing concentrations of gold-silver alloy nanoparticles of sample A, B and C are reported in Table
ST7, ST8 and ST9 respectively (See
Supplementary data).
It is evident from the Table S7, S8 and S9 that the thermal diffusivity and fluorescence emission intensity values were decreased with increase in concentration of nanoparticles incorporated into the eosin B dye where as the thermal lens signal amplitude was found to be enhanced. The reduction in fluorescence emission intensity in turn contributed to the enhancement of TL signal.
As the power of the pump beam was kept constant throughout the experiment, the determining features that contributed to the enhancement of thermal lens signal was the improvement in the ratio of non-radiate to radiative emission of the dye-nanoparticle system due to fluorescence quenching of the dye in the presence of nanoparticles and the broadening of absorption spectrum of the dye with introduction of nanoparticles [36]. The reduction in the thermal diffusivity of the dye with addition of plasmonic nanoparticles can be explained on the basis of cluster formation between nanoparticles and dye molecules [37, 34]. Even though there is possibility of effective heat transfer within the cluster, the heat exchange with the base fluid is negatively affected. The heat diffusion between dye molecules and base fluid turned slower due to this clustering effect. A Comparison of the effect of silver, gold and gold –silver alloy nanoparticles (samples G, S and A, B, C) on the thermal lens signal decay of the eosin B dye is illustrated in Figure 11. Corresponding values of thermo-optical parameters like thermal diffusivity D, characteristic time constant tc and fluorescence emission intensity is given as table 3.
Table 3 Thermo-optical parameters of eosin B with silver, gold and gold-silver alloy nanoparticles.
Sample Name
Time constant
Thermal
Thermal lens
Fluorescence
tc (s)
diffusivity
signal amplitude
emission
-8
2
D (10 m /s)
intensity
D
0.0584
6.257
0.04132
987.43
D+S
0.1914
1.909
0.11628
808.73
D+A
0.1300
2.811
0.10484
798.66
D+B
0.1120
3.262
0.09067
721.15
D+C
0.1074
3.402
0.09434
739.15
D+G
0.0923
3.958
0.08333
656.79
Figure 11 (a) The normalized intensity decay of the probe beam with time of eosin B dye samples with gold, silver and gold-silver alloy nanoparticles. (b) Amplitude of the thermal lens signal and thermal diffusivity of eosin B dye samples with gold, silver and gold-silver alloy nanoparticles.
It was observed that the amplitude of thermal lens signal was further enhanced by changing the composition of the nanoparticle samples incorporated into the system. With the blue shift in SPR peak from samples G to S, the thermal lens signal amplitude increased. The major factor contributing to the enhancement is the broadening of absorption spectrum of dye with the introduction of nanoparticle and the fluorescence quenching by nanoparticles. The absorbance at the pump beam wavelength was considerably improved by SPR tuning which in turn resulted in amplified thermal lens signal and faster thermal decay.
The thermal diffusivity was found to be highest for dye alone samples. With the introduction of plasmonic nanoparticles into the system, an improvement in the decay rate or reduction in thermal diffusivity was
observed which depends on the fluorescence quenching efficiency and absorbance enhancement. This leads to the possibility of fine tuning of the thermo optical properties of eosin B dye with nanoparticles. Further reduction in thermal diffusivity can be achieved with the surface plasmon tuning towards pump beam wavelength using silver-gold alloy nanoparticles.
4. CONCULSION
The influence of plasmonic nanoparticles on the thermo optical properties of eosin B dye was investigated. With the introduction of laser ablated gold, silver and gold-silver alloy nanoparticles to the eosin B dye solution, the thermal diffusivity and fluorescence emission intensity of the samples were decreased where as the thermal lens signal was found to be enhanced. The reduction of thermal diffusivity and amplification of TL signal was observed to be enlarged with the concentration of nanoparticles incorporated into the dye solution. The improvement in the ratio of non-radiate to radiative emission of the dye-nanoparticle system due to fluorescence quenching of the dye in the presence of nanoparticles and the broadening of absorption spectrum of the dye with introduction of nanoparticles have major role in the enhancement of thermal lens signal. The variation of thermal diffusivity, thermal lens signal amplitude and fluorescence emission intensity with surface plasmon peak tuning was analyzed using gold-silver alloy nanoparticles. It was found that further reduction of thermal diffusivity and enhancement of TL signal is possible by tuning SPR band with gold-silver alloy nanoparticles. Hence the present study concludes that fine tuning of thermo optical properties of eosin B dye can be made possible by varying the nanoparticle concentration and composition. The investigations on thermo-optical properties of eosin B incorporated with plasmonic nanoparticles have significance in photothermal applications of the dye like lasing, laser induced fluorescence studies, photo thermal imaging and improving the sensitivity of the fluorescence sensing systems.
Acknowledgement
The first author acknowledges the financial support from CSIR for research funding.
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Thermo-optical properties of Eosin B dye can be tuned with laser synthesized gold-silver alloy nanoparticles. A reduction in thermal diffusivity and enhancement in thermal lens signal of eosin B as with incorporation of gold-silver alloy nanoparticles. Effective tuning by changing composition and concentration of nanoparticles.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.