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Time resolved emission studies of Ag-adenine-templated CdS (Ag/CdS) nanohybrids

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2009 Nanotechnology 20 095703

(http://iopscience.iop.org/0957-4484/20/9/095703)

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Nanotechnology 20 (2009) 095703 (10pp) doi:10.1088/0957-4484/20/9/095703

Time resolved emission studies ofAg-adenine-templated CdS (Ag/CdS)nanohybridsAnil Kumar and Vidhi Chaudhary

Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee- 247667, IndiaandCentre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee-247667, India

E-mail: anilkfcy@iitr.ernet.in

Received 27 October 2008, in final form 17 December 2008Published 11 February 2009Online at stacks.iop.org/Nano/20/095703

AbstractAg-adenine-templated CdS (Ag/CdS) nanohybrids have been synthesized and characterized bytransmission electron microscopy, selected area electron diffraction, x-ray diffraction, andoptical, fluorescence and time resolved emission spectroscopy. Adenine serves as an effectivematrix for the stabilization of Ag/CdS through interaction of N(1), N(3) and –NH2 with Ag.The amount of Ag in the nanohybrid is observed to influence the organization of the Ag andCdS phase in the composite and also modifies the nature of electronic transition in CdS. For thenanohybrid containing a molar ratio of 0.1 of Ag/ CdS, CdS nanoparticles (2.5 nm) surroundthe Ag (6.5 nm) core. The excitation of these particles by 340 nm light, where the absorptiondue to the Ag phase in the nanohybrid is negligibly small, results in the enhancement offluorescence by a factor of 7 compared to that of bare CdS. For the particles containing a molarratio of Ag/CdS of unity, bigger clusters (14 nm) are produced causing the quenching ofemission of CdS. In time resolved emission spectroscopy the spectral shift from 415 nm(3.0 eV) to 550 nm (2.26 eV) monitored over a period of 1–220 ns is understood by therelaxation of charge within the surface states of varied energy from 180 to 370 eV. The observedchanges in fluorescence behavior in terms of intensity, lifetime and spectral shift are understoodin terms of electronic interaction between Ag and CdS phases. The manipulation of electronicand fluorescence properties in these nanohybrids could be exploited for optoelectronic,molecular-recognition and sensing applications.

S Supplementary data are available from stacks.iop.org/Nano/20/095703

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Surface enhanced Raman scattering (SERS), optical, optoelec-tronic and fluorescence properties for a variety of dyes andother molecules are known to be significantly enhanced whenpresent in the vicinity of a metallic surface [1–10]. Such en-hancement of the properties has allowed the detection of even asingle molecule with high sensitivity and specificity, which hastremendous applications in the areas of environmental science,biology, imaging and medicine [6, 11–15]. Replacementof fluorescent organic dye(s) by fluorescing semiconductorquantum dots makes it more interesting because of the

possibility of tuning their size, shape, optical, fluorescenceand electronic properties, and obtaining relatively higherphotostability [16–20].

The functional nature and well-defined structural proper-ties of biomolecules have recently been extensively exploitedas templates for the fabrication of new nanomaterials, whichmay be used as building blocks for making self-assembliesand devices [21–26]. Interfacing of either quantized metalor semiconductor nanoparticles with biomolecules havingsimilar dimensions has been reported to produce nanohybridassemblies with entirely new optical, electronic and photonicproperties [21, 24–32]. It would be interesting to move a step

0957-4484/09/095703+10$30.00 © 2009 IOP Publishing Ltd Printed in the UK1

Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

further by making nanohybrids comprising both the biotem-plated nanosized fluorescing semiconductor and colloidalmetal nanoparticles in efforts to further enhance their abovementioned properties. Selecting the experimental conditions,such that the optical absorption due to the fluorophore is notinfluenced by the presence of metal nanoparticles, might beinteresting with regard to the investigation of the effect ofmetals on electronic transitions in the fluorophore. Relaxationkinetics of charge carriers can be used to probe their electronicproperties.

In the present work we have undertaken an investigationof the synthesis of nanohybrids on silver metal with adenine-templated CdS. The optical, fluorescence, electronic andtime resolved fluorescence behaviors of these materials havebeen analyzed for an understanding of these systems. Suchmulticomponent nanohybrids may find potential applicationsin photonic devices and chemical/biochemical sensing usingfluorescent probes.

2. Experimental details

2.1. Reagents

Cadmium perchlorate, adenine, dialysis membrane, eosin(Aldrich), hydrochloric acid (Qualigens), sodium citratetribasic dihydrate (SRL), silver nitrate, sodium hydroxide,perchloric acid and sodium borohydride (Merck) were ofanalytical grade and were used without any further purification.Nitrogen (>99.9% purity) was procured from Sigma. Dialysiswas performed by using Spectra/Por membrane (MW 6000–8000).

2.2. Equipment

Optical and fluorescence spectra were recorded using aShimadzu UV-2100S spectrophotometer and a Shimadzu RF-5301-PC spectrofluorophotometer, respectively. Electronmicrographs and selected area diffraction (SAED) patternswere measured on a FEI-Philips Morgagni 268D and a FEI-Tecnai 200 kV Digital TEM having variable magnificationup to 280 000× and 1100 000×, respectively. The size ofthe nanoparticles in different TEM images was determinedusing digital micrograph software. IR spectra of solid sampleswere obtained on a Thermo Nicolet Nexus Fourier transforminfrared spectrophotometer. X-ray diffraction (XRD) patternsof powder samples were recorded on a Philips DW 1140/90x-ray diffractometer using the Cu Kα line of the x-raysource. The fluorescence lifetimes were measured on a HoribaJobin-Yvon Fluorescence Lifetime System using NanoLEDsand LDs as excitation sources. Fluorescence decay curveswere analyzed kinetically by DAS6 software from IBH. Thegoodness of the fit was determined by evaluating χ2 from thefitted plots.

Time resolved emission spectra (TRES) were recordedat different time intervals as determined from their respectivefluorescence decay curves. The TRES measurement accessoryprovided a three-dimensional data set, which was slicedorthogonally to the time axis to produce 2D spectra of countsversus wavelength.

2.3. Methodology

2.3.1. Preparation of colloidal silver. Colloidal Agnanoparticles were synthesized using an approach very similarto that used for the preparation of gold particles [33], withsome minor modifications. Colloidal suspensions of silver seedparticles were prepared by mixing of 0.5 ml aqueous AgNO3

(1%) in 100 ml of H2O under vigorous stirring followed bythe addition of 0.5 ml of trisodium citrate (1%) and 0.5 mlof NaBH4 (0.075%) to 1% trisodium citrate sequentially at aninterval of about 1 min. This solution was stirred for about5 min and stored at 5 ◦C as stock solution. Colloidal silverparticles were then prepared by adding 0.2 ml of 1% AgNO3

to 100 ml of boiling doubled distilled water followed by theaddition of 0.2 ml of the ‘seed’ colloids and 0.2 ml of the1% trisodium citrate solution sequentially. This mixture wasrefluxed for about 10 min and was then cooled down slowlyat room temperature. The resulting colloidal solution wasdialyzed against water for about 2 h for the removal of excesscitrate and Ag+ ions.

Adenine-capped Q-CdS was prepared by adding 5 ×10−5 mol dm−3 of freshly prepared SH− to the de-aerated1 × 10−4 mol dm−3 aqueous Cd(ClO4)2 containing 1 ×10−3 mol dm−3 adenine at pH 11.0 using a previously reportedliterature method [25]. As noted earlier, these nanoparticleswere unstable at lower pH and underwent rapid coagulation.At pH ∼ 9.0, precipitation was observed to take place with infew hours.

2.3.2. Preparation of colloidal Ag-adenine-templatedCdS (Ag/CdS) nanohybrids. Ag-adenine-templated CdSnanohybrids (Ag/CdS) were synthesized by adding variousamounts of colloidal Ag to the degassed Cd2+ solution (1 ×10−4 mol dm−3) containing 1 × 10−3 mol dm−3 adenine priorto the addition of SH−. The reaction mixture was degassed bypurging N2 strongly for about 15 min followed by the injectionof SH− (5×10−5 mol dm−3), which results in the formation ofAg/CdS nanohybrids. The pH at each stage of the experimentwas fixed at 11.0. In these solutions molar ratios of Ag to CdSwere varied from 0.025 to 1. It may be mentioned that bothhigh pH and excess of Cd2+ are required for the synthesis ofAg/CdS nanohybrids. At a pH of 9.0, this colloidal solutionunderwent coagulation in <12 h.

Transmission electron micrographs of different colloidswere recorded by applying a small drop of colloidal solutionto the carbon coated copper G-200 grid. Copper grid was driedin the dark prior to its examination. The particle size wasmeasured using a soft imaging system from SIS Olympus.

3. Results

3.1. Optical spectra

Figure 1 presents the absorption spectra of adenine-cappedCdS containing various amounts of Ag. An examination ofthese spectra reveals that at a low molar ratio of Ag to CdSthe absorption spectrum of nanohybrid is similar to the sumspectra obtained by simple addition of the absorption due toits individual components, i.e. Ag and CdS (supplementary

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

Figure 1. Optical spectra of Ag-adenine-templated CdS nanohybrids containing various molar ratios of Ag to CdS. Different curves shown inthe above figure: sum spectra ( ); fresh sample (– – –); aged sample (· · ·).

data, figure S1 (available at stacks.iop.org/Nano/20/095703))except that the absorption in the visible range beyond 450 nmis slightly less. An increase in the molar ratio of Ag to CdS to0.25 results in a decrease in the absorption in the range 330–550 nm and an increase at higher wavelengths. At a molarratio of unity the decrease in absorption becomes significant inthe vicinity of the plasmon band (420 nm). The absorbanceis, however, increased at higher energy in the UV region(>3.77 eV).

3.2. Transmission electron microscopy

TEM images of Ag-adenine-capped CdS and adenine-mediated Ag/CdS nanohybrid having a molar ratio of Ag toCdS of 0.1 are depicted in figure 2. The size histogramof adenine-templated Ag/CdS hybrid showed two types ofparticle with an average size (size distribution) of 2.5 nm (1–5 nm) and 6.5 nm (5–9 nm), respectively. An examinationof these images reveals that bare Ag particles are sphericalin shape, whereas adenine-mediated CdS nanoparticles exhibitcoiled aggregates. In Ag/CdS nanohybrids at low Ag aspecific distribution of particles is seen in which smallerparticles (2.5 nm) surrounded the bigger dark particles (6.5 nm)(figure 2(c)). For higher silver content such distribution couldnot be observed as there might possibly be aggregates of silverwith adenine along with the Ag/CdS nanohybrid (not shown).

SAED patterns due to Ag, CdS and adenine-templatedAg/CdS nanohybrids are shown in figure 3A. SAED patterns

of colloidal silver and CdS particles exhibit ring patterns.Indexing of these rings shows that each silver and adenine-capped CdS particle is produced in the hexagonal phase with dvalues (A): 2.00 (103), 1.76 (104), 1.21 (202) and 1.00 (0010),and 2.43 (102), 2.06 (110), 1.88 (103), 1.51 (104), 1.39 (203),1.11 (006), respectively. The adenine-templated Ag/CdShybrid showed rather different arrays of diffraction spots onthe top of the rings (figure 3A(c)). Indexing of these patternsshows that the composite contains planes corresponding to CdS(3.14 (101), 2.43 (102), 1.57 (202), 1.39 (203), 1.25 (212)),Ag (2.42 (101), 2.00 (103), 1.56 (105), 1.38 (112), 1.24 (201),1.11(108)) and Cd(OH)2 (1.11 (121), 1.09 (104)) in hexagonalphases. SAED patterns at higher Ag/CdS ratio (�0.25) exhibitmore prominent rings due to silver nanoparticles (not shown).

3.3. XRD analysis

The structural properties of silver and the CdS phase in thehybrid were also examined by recording the XRD patterns ofthe powder sample (figure 3B). Indexing of these patternssupports the presence of both silver and CdS in hexagonalphases in the hybrid as arrived above by SAED analysis.

3.4. IR spectra

IR spectra of pure adenine, adenine-supported CdSand adenine-templated Ag/CdS nanoparticles arepresented as supplementary data (figure S2 (available at

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

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Figure 2. TEM images and size histograms of silver nanoparticles (a), (a′); pure adenine-capped CdS (b), (b′); Ag-adenine-templated CdSnanohybrids having a molar ratio of Ag to CdS 0.1 (c), (c′), (c′′). Encircled region exhibits the surrounding of bigger dark particles by smallerparticles.

stacks.iop.org/Nano/20/095703)) and their spectral data arebeen summarized in table 1. The IR spectrum of pure adenineexhibits all its characteristic peaks as reported previously [34].CdS-supported adenine shows specific interaction with bothpyrimidine and imidazole rings in general, as demonstrated bya shift in their respective deformation frequencies. Besides,it depicts an interaction with –NH2 as seen by the loweringof its bending vibration frequency from 1666 to 1629 cm−1

in agreement with the observations made in the previousstudy [25]. The IR spectrum of adenine-templated Ag/CdSnanohybrids having a molar ratio of Ag to CdS of 0.1 exhibitsseveral additional changes with respect to the spectrum of

adenine-mediated CdS, as a result of interaction of Ag withadenine via the N (1) and N (3) of the pyrimidine rings, and–NH2, as indicated by the disappearance of peaks due tothese moieties (see table 1 and supplementary data figure S2(available at stacks.iop.org/Nano/20/095703)). Peaks due toother functional groups, however, did not show any changes intheir absorption frequencies.

3.5. Fluorescence spectra

Figure 4 displays the emission spectra of adenine-cappedCdS and adenine-templated Ag/CdS nanohybrids containing

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

a b

c

Figure 3A. SAED of silver nanoparticles (a), adenine-capped CdS(b) and Ag-adenine-templated CdS nanohybrids containing a molarratio of Ag to CdS 0.1 (c) recorded from images given in figure 2 (a),(b) and (c), respectively. Indexing of the SAED patterns of Ag/CdSnanohybrids shown in (c): CdS (A(101), B(102), D(202), E(203),F(212)); Ag (B(101), C(103), D(105), E(112), F(201), G(108)) andCd(OH)2 (G(121), H(104)).

1000

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Figure 3B. X-ray diffraction patterns of Ag-adenine-templated CdSnanohybrids containing a Ag to CdS molar ratio of 0.1.

various molar ratios of Ag to CdS (0.025–1). By using theexcitation wavelength (340 nm), where the absorption of silveris negligible compared to that of CdS (supplementary data,figure S1 (available at stacks.iop.org/Nano/20/095703)), theeffect of Ag on the emission behavior of CdS has been probed.The fluorescence intensity due to CdS is enhanced gradually,accompanied by a small blue shift in the emission maxima by

Ag/CdSa - 0.0b - 0.025c - 0.1d - 0.25e - 1.0

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Figure 4. Emission spectra of Ag-adenine-templated CdSnanohybrids containing various molar ratios of Ag to CdS. Inset:relative fluorescence intensity as a function of the molar ratio of Agto CdS. Curve e has been normalized with absorption data.λex = 340 nm.

Table 1. IR spectral data of adenine, adenine-capped CdS andAg-adenine-templated CdS nanohybrids.

Vibrationmode/functionalgroup

Adenine(cm−1)

Adenine-capped CdS(cm−1)

Ag-adenine-templatedCdS nanocomposite(cm−1)

NH2 (bending) 1666 1629 1594C6N1 1474 1450 —C4N9 1419 1421 —N9H 1384 1384 1384N9C8 1352 1352 1352C2N3 1309 1310 —C4N9 1127 1120 1115C2H 943 947 —Imidazole ring(bending)

914 906 —

Pyrimidine ring(deformation)

804 800 —

N3C4 723 722 —Imidazole ring(deformation)

640 638 630

NH2 (twist) 534 538 —

increasing the molar ratio of Ag/CdS up to 0.1. However, areduction in pH from 11.0 to 9.0 reduced the emission intensityby about an order of magnitude. A further increase in themolar ratio to 0.25, however, resulted in a decrease in theintensity of emission and the emission maximum gets slightlyred shifted (figure 4, curve e). At still higher molar ratios, theemission intensity is reduced drastically, and at a molar ratio ofunity it becomes very similar to that of bare CdS to which nosilver was added. These findings suggest a complex interactionbetween the irradiated CdS and Ag, which could be probed bymonitoring the relaxation kinetics of charge carriers due to CdSin different environments produced by the presence of variousamounts of silver.

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

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Figure 5A. Fluorescence decay curves of fresh (X) and aged (Y) Ag-adenine-templated CdS nanohybrids containing different molar ratios ofAg to CdS: (a) 0.0, (b) 0.025, (c) 0.1, (d) 1.

Table 2. Lifetimes of Ag-adenine-templated CdS nanohybrids having different Ag to CdS molar ratios (λex = 340 nm; λem = 540 nm).

Component 1 Component 2 Component 3Molar ratioof Ag to CdS τ1 (ns) Emission (%) τ2 (ns) Emission (%) τ3 (ns) Emission (%) τ (ns) χ2

(a) Fresh samples

0.00 0.99 16.96 19.6 28.25 156 54.79 91.3 1.2(0.19) (0.016) (0.003)

0.025 1.03 11.22 21.7 21.22 196 67.55 137 1.1(0.18) (0.016) (0.005)

0.05 1.11 6.27 25.6 16.02 223 77.71 177 1.1(0.16) (0.018) (0.010)

0.10 1.26 5.25 29.02 16.21 226 78.54 182 1.1(0.19) (0.020) (0.016)

0.25 1.03 5.42 27.7 17.27 228 77.32 181 1.1(0.17) (0.020) (0.011)

0.50 0.98 6.18 24.9 18.07 216 75.75 168 1.1(0.17) (0.020) (0.009)

1.00 0.88 8.54 21.5 19.01 213 72.45 158 1.2(0.19) (0.017) (0.006)

(b) Aged sample

0.00 1.10 18.24 19.4 28.33 148 53.43 85 1.2(0.18) (0.016) (0.004)

0.025 1.16 11.27 23.1 21.20 202 67.53 141 1.2(0.17) (0.016) (0.006)

0.05 1.13 6.24 27.4 16.95 231 76.82 182 1.1(0.17) (0.019) (0.010)

0.10 1.18 5.12 28.6 15.74 237 79.15 192 1.1(0.16) (0.020) (0.012)

0.25 1.07 5.52 27.3 17.23 228 77.25 181 1.1(0.17) (0.021) (0.011)

0.50 1.05 6.95 26.0 19.46 221 73.60 168 1.1(0.97) (0.019) (0.008)

1.00 0.97 10.57 19.6 19.88 205 69.55 147 1.2(0.17) (0.016) (0.005)

3.6. Time resolved emission spectroscopy

To analyze the charge carrier dynamics in Ag/CdS nanohybrid,time resolved emission spectra of the illuminated samplesconsisting of varied molar ratios of Ag/CdS along withthat of bare adenine-capped CdS were recorded on differenttimescales in the wavelength range of 400–750 nm. Time

intervals for recording the transient spectra were worked outfrom the fluorescence decay curve fitting data for the respectivetrace. Each component was selected from the time slicewindow when the relative emission intensity was maximalcorresponding to that component.

The fluorescence decay curves due to these samplesfollowed three-exponential decay kinetics at all wavelengths

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

Figure 5B. Time resolved emission spectra (TRES) ofAg-adenine-templated CdS nanohybrids containing different molarratios of Ag to CdS.

having time constants corresponding to ns, tens of ns andhundred(s) of ns, respectively. Lifetime traces recorded uponexcitation by 340 nm radiation are shown in figure 5A and thecorresponding data are included in table 2. An increase in theamount of Ag in the composite initially enhances the lifetimesof all the three components up to a molar ratio of Ag/CdS of0.1, and thereafter there was a decrease in the lifetimes.

It may be pointed out the fluorescence monitoredupon the excitation corresponding to the plasmonic bandof Ag (>400 nm) results in the enhancement of emis-sion intensity associated with a decrease in fluores-cence lifetime (supplementary data, figure S4 (available at

stacks.iop.org/Nano/20/095703)). For a typical case of agedparticles having a molar ratio of Ag to CdS of unity (τ1 =0.10 ns (78.44% emission); τ2 = 3.79 ns (4.99% emission);τ3 = 109 ns (16.57% emission)) the average lifetime (〈τ 〉) isreduced drastically to 18 ns.

An examination of the time resolved emission spectra(figure 5B) reveals that the relative emission intensity of the415 nm band after about 1 ns is enhanced with increasingsilver component in the Ag/CdS nanohybrid and the emissionmaximum is gradually shifted to higher energy. For theparticles containing a molar ratio of Ag/CdS of unity, theintensity at the 415 nm band is increased by a factor of 3compared to that of pure CdS and the emission maximum isshifted from 415 nm (3.0 eV) to 410 nm (3.03 eV). The changeat 415 nm is, however, associated with a complex change inemission behavior at longer wavelengths, where the intensityat high Ag was the lowest. For the spectrum recorded after15 ns of irradiation, the emission maximum shifts to a longerwavelength at 550 nm (2.26 eV). On this timescale the relativeemission intensity is the highest for particles having a molarratio of Ag/CdS of 0.1 and the lowest for particles havinga higher molar ratio of Ag/CdS of unity. This spectrum isfurther transformed to yield a new spectrum over a period of220 ns (figure 5B(Z)). The intensity of the band correspondingto Ag/CdS of 0.1 was still the highest, whereas it is reducedto almost zero for particles having a molar ratio of Ag/CdSof unity, which is very similar to that of pure adenine-cappedCdS.

Interestingly, the fluorescence lifetime is observed toincrease with decreasing emission energy (table 3). Thisincrease was apparent for all the three components in differenttime domains (supplementary data, figure S3 (available atstacks.iop.org/Nano/20/095703)).

Three-dimensional representations of the counts (fluo-rescence intensity) as a function of channels (time) andwavelengths for all these samples are shown in figure 5C,which essentially manifests very similar observations to thosepresented by the combined spectra of these samples ondifferent timescales shown in figure 5B.

4. Discussion

The formation of adenine-mediated Ag/CdS nanohybrid isindicated by a change in the optical spectrum of the composite,which is fairly different from the sum spectrum due to itscomponent silver and CdS recorded separately (figure 1). Thepresence of Ag, CdS and Cd(OH)2 phases in the hybrid isindicated both by the analysis of the SAED (figure 3A(c))and by the XRD (figure 3B) measurements. An interactionbetween the two phases in the hybrid is also evidenced bythe TEM image in which the addition of a small amount ofAg does not allow a chain of CdS clusters to grow. It ratherproduced isolated CdS clusters surrounding the larger Agnanoparticles, suggesting that at low Ag contents the bindingof Ag to the adenine matrix of CdS particles prevents theirsecondary growth via weak van der Waals forces to form biggeraggregates. IR spectroscopic data (table 1 and supplementarydata, figure S2 (available at stacks.iop.org/Nano/20/095703))

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

Time Calibration - 1.211x10-10s/ch Time Calibration - 1.211x10-10s/ch

Time Calibration - 1.211x10-10s/ch

a b

c

Figure 5C. Three-dimensional time resolved emission spectra (TRES) of Ag-adenine-templated CdS nanohybrids containing different molarratios of Ag to CdS: (a) 0.0, (b) 0.1, (c) 1. 8000 channels correspond to 968 ns.

revealed the nature of bonding interaction of Ag with CdSnanoparticles via different sites of the adenine matrix. Theinstability of the hybrid at low pH suggests the involvementof Cd(OH)2 in stabilizing the structure of the Ag/CdSnanohybrid. Thus, based on the above findings the structureof Ag/CdS nanohybrids having a molar ratio of 0.1 can bedescribed as given in scheme 1.

In contrast, for larger amounts of Ag bigger clusters areproduced without any distinction between the two phases,which is explained as due to the reduced internuclearseparation between the two phases which thereby induces theformation of relatively larger clusters.

In order to examine the role of the matrix, adenine wasreplaced by 6-dimethylaminopurine (6-DMAP) to stabilizethe CdS particles. Ag/CdS nanohybrid containing 6-DMAP-capped CdS particles exhibits a significant increase in the〈τ 〉 to 330 ns. In steady state experiments, the fluorescencequantum efficiency due to these particles was also observed tobe enhanced by a factor of 1.5 compared to that of Ag/CdSnanohybrid containing adenine-mediated CdS. These changesin the electronic properties can be assigned to the differencein the nature of the binding of metal to various sites onthe biomolecule and the structure of matrix of the particle.This experiment clearly reveals the role of the matrix in thesynthesis and control of the electronic properties of thesematerials. Details of this work will be published elsewhere.

Scheme 1.

The steady state fluorescence spectra exhibit complexchanges in emission behavior as a function of the added silverin the hybrid (figure 4), i.e. at a molar ratio of Ag/CdS

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Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

Table 3. Effect of emission energy on the lifetime of Ag-adenine-templated CdS nanohybrids having different Ag to CdS molar ratios(λex = 340 nm).

Component 1 Component 2 Component 3

λem (nm) τ1 (ns) Emission (%) τ2 (ns) Emission (%) τ3 (ns) Emission (%) τ (ns) χ2

(a) Ag/CdS molar ratio = 0.0

460 0.93 37.47 7.83 29.03 79.5 33.50 29.2 1.0(0.32) (0.029) (0.003)

540 0.99 16.96 19.6 28.25 156 54.79 91.3 1.2(0.19) (0.016) (0.003)

640 1.41 11.54 24.8 24.12 194.5 64.34 131 1.2(0.29) (0.034) (0.018)

(b) Ag/CdS molar ratio = 0.1

460 0.929 27.57 10.3 24.05 108.9 48.38 55.4 1.2(0.31) (0.02) (0.004)

540 1.26 5.25 29.02 16.21 226 78.54 182 1.1(0.19) (0.02) (0.016)

640 1.32 6.93 29.64 18.56 244 74.50 187 1.1(0.14) (0.01) (0.008)

(c) Ag/CdS molar ratio = 1.0

460 0.723 38.25 7.93 25.03 96.38 36.72 37.6 1.2(0.37) (0.02) (0.002)

540 0.881 8.54 21.5 19.01 213 72.45 158 1.2(0.19) (0.017) (0.006)

640 0.966 4.40 26.02 17.4 222 78.16 178 1.1(0.15) (0.02) (0.018)

of 0.1 the fluorescence peak due to CdS gets blue shiftedand the intensity of emission is enhanced by a factor ofabout 7 corresponding to a quantum yield of emission of0.028. In contrast, at a molar ratio of Ag/CdS of unity, thefluorescence peak due to CdS shifts to the red and the intensityof emission is reduced to become very similar to that of pureCdS (� = 0.004). We can understand these observationsby the fact that the presence of metal in the nanohybrid mayinfluence both radiative and non-radiative transition dependingon its concentration. The presence of Ag near the surface ofCdS possibly induces a local field, which may enhance thedensity of excited states in the fluorophore. At low Ag moreshallow traps located at higher energy are created in the hybridbecause of the binding of Ag to the adenine matrix of CdSparticles. These act as radiative centers, in comparison to thesituation in the bare adenine-capped CdS, thus resulting inan increase in the quantum yield of fluorescence associatedwith a blue shift in emission. At high Ag relatively moredeeper traps are generated, most of which introduce non-radiative centers causing a decrease in quantum efficiency offluorescence exhibiting red shifted emission. If these particlesare illuminated by light of a wavelength corresponding to theplasmonic absorption due to Ag, lifetime data indicate thatthe oscillating electrons initially tend to populate the shallowtraps at high molar ratios of Ag/CdS, unlike the situationobserved under irradiation by 340 nm light, and relax rapidlyto the deeper traps within the band gap of CdS enhancingnon-radiative transition. Interestingly, the observation of theenhancement of emission intensity associated with a decreasein fluorescence lifetime is very similar to the reports byLakowicz and co-workers [4] and is not discussed any further.

The spectral transformation recorded from TRES mea-surements (figure 5B) suggests a relaxation of charge over aperiod of less than a nanosecond to a few hundred nanosecondswithin the surface states of various energies located in theband gap of CdS. An increase in the intensity of emission atthe 415 nm band associated with a blue-shifted emission asa function of the concentration of Ag in the composite canbe understood by the injection of electrons from excited CdS(−1.0 V versus NHE) [35] to the Fermi band of Ag (Efb =0.15 V) [36] in accordance with the thermodynamic driveindicated by their respective electrode potentials. However,a relatively faster decay of fluorescence at 415 nm (3.0 eV)compared to that of lower energy at 460 nm (2.7 eV), 540 nm(2.3 eV) and 640 nm (1.94 eV) (table 3 and supplementarydata, figure S3 (available at stacks.iop.org/Nano/20/095703)),suggests that electrons in the shallow traps do not stay for verylong and relax to deeper traps of various energies over a periodof 1–220 ns, resulting in the quenching of emission associatedwith the red-shifted fluorescence maxima (figure 5B). Fromthe lifetime data, the distribution of traps in different energystates is estimated to lie in between 180 and 370 eV.

The enhancement of charge relaxation with increasingmolar ratio of Ag/CdS in the hybrid (table 2) indicates theexistence of shallow and deeper traps, which are interlinkedto each other at the interface of CdS nanoparticles. Theenergy dependent change in the fluorescence decay due toCdS suggests almost a continuous distribution of surfacestates of various energies (180–370 eV) within the band gapof CdS (table 3, supplementary data, figure S3 (availableat stacks.iop.org/Nano/20/095703)). Evidently, there isequilibrium between the surface states of different energies,

9

Nanotechnology 20 (2009) 095703 A Kumar and V Chaudhary

in which the distribution of the population of shallow anddeeper traps is controlled kinetically by the amount of Ag inthe hybrid. Therefore, initially more shallow traps will becomepopulated early on.

At low molar ratios of Ag/CdS:

CdS + Ag � (CdS − Ag)hν−−−−−→

(∼340 nm)(CdS∗ − Ag) (i)

(CdS∗ − Ag) � [CdS∗(e−h+)- - - - - -Ag]� [CdS(h+)- - - - - -(Ag−)S]. (ii)

The injection of the electron to the shallow traps is drivenby the amount of Ag in the hybrid. An increase in Ag in thehybrid may enhance the density of shallow traps by pushing theequilibrium in the forward direction as shown in equation (iii):

(iii)

At high Ag the charge from shallow traps relaxes rapidly tooccupy the deeper traps:

[CdS(e−h+)- - - - - -(Agn)S] � [CdS(h+)- - - - - -(Ag−n )D]

(iv)where S and D denote shallow and deep traps, respectively.

(n − 1) Ag represents excess silver.From the TRES data it is thus apparent that the silver

nanoparticles added in the vicinity of CdS introduce variousdensities of traps in the interfacial region, the nature of whichcould be controlled kinetically by optimizing the amount ofAg in the nanohybrids as per equations (i)–(iv). A fraction ofthe separated charge carriers thus generated in shallow and/ordeeper traps may undergo radiative recombination to exhibitthe observed emission on different timescales.

5. Conclusion

In summary, adenine effectively mediates the formation ofAg/CdS nanohybrids. The binding of Ag to the adeninematrix of the particles attached through Cd(OH)2 passivatesits surface favorably to enhance the separation of chargeand thereby the quantum efficiency of emission. Thepresence of Ag modifies electronic transitions in CdS byintroducing surface states of different energy in the interfacialregion. TRES measurements demonstrate a variation inthe dynamics of charge carriers as a function of theamount of Ag in the hybrid, providing evidence for anelectronic interaction between Ag and adenine-mediated CdSphases. The observation of an enhanced fluorescenceresponse, fluorescence lifetime and a time resolved spectralshift recorded under specific experimental conditions uponexcitation by the light, largely absorbed by fluorophoresin the nanohybrid, provides an important tool to exploitthese materials for photonic, molecular-recognition and sensorapplications.

Acknowledgments

Financial support of this work by CSIR, New Delhi isgratefully acknowledged. VC is grateful to CSIR, New Delhifor the award of a SRF. Thanks are also due to the Director,AIIMS, New Delhi and Head, IIC, IITR, Roorkee for providingus with the TEM, XRD and single photon counter facilities.

References[1] Lecomte S, Matejka P and Baron M H 1998 Langmuir 14 4373[2] Pan S, Wang Z and Rothberg L J 2006 J. Phys. Chem. B

110 17383[3] Geddes C D, Cao H, Gryczynski I, Gryczynski Z, Fang J and

Lakowicz J R 2003 J. Phys. Chem. B 107 3443[4] Geddes C D, Aslan K, Gryczynski I and Lakowicz J R 2006

Fluorescence Sensors and Biosensors 2nd edn,ed R B Thompson (New York: CRC/Taylor and Francis)p 121

[5] Zhang J, Gryczynski I, Gryczynski Z and Lakowicz J R 2006J. Phys. Chem. B 110 8986

[6] Tong L, Li Z, Zhu T, Xu H and Liu Z 2008 J. Phys. Chem. C112 7119

[7] Chen J-W, Liu X-P, Feng K-J, Liang Y, Jiang J-H,Shen G-L and Yu R-Q 2008 Biosens. Bioelectron. 24 66

[8] Tang Z, Kotov N A and Giersig M 2002 Science 297 237[9] Pompa P P, Martiradonna L, Torre A D, Carbone L,

Mercato L L D, Manna L, Vittorio M D, Calabi F,Cingolani R and Rinaldi R 2007 Sensors Actuators B126 187

[10] Yang Y, Shi J, Tanaka T and Nogami M 2007 Langmuir23 12042

[11] Kim K, Park H K and Kim N H 2006 Langmuir 22 3421[12] Fu Y and Lakowicz J R 2006 J. Phys. Chem. B 110 22557[13] Jiang J, Bosnick K, Maillard M and Brus L 2003 J. Phys.

Chem. B 107 9964[14] Shimizu K T, Woo W K, Fisher B R, Eisler H J and

Bawendi M G 2002 Phys. Rev. Lett. 89 117401–1[15] Chen Y, Munechika K and Ginger D S 2007 Nano Lett. 7 690[16] Neretina S, Qian W, Dreaden E, El-Sayed M A, Hughes R A,

Preston J S and Mascher P 2008 Nano Lett. 8 2410[17] Kumar A and Chaudhary V 2007 J. Photochem. Photobiol. A

189 272[18] Yang J, Elim H I, Zhang Q B, Lee J Y and Ji W 2006 J. Am.

Chem. Soc. 128 11921[19] Wang Y, Li M, Jia H, Song W, Han X, Zhang J, Yang B,

Xu W and Zhao B 2006 Spectrochim. Acta A 64 101[20] Kamat P V and Shanghavi B 1997 J. Phys. Chem. B 101 7675[21] Storhoff J J and Mirkin C 1999 Chem. Rev. 99 1849[22] Torimoto T, Yamashita M, Kuwabata S, Sakata T, Mori H and

Yoneyama H 1999 J. Phys. Chem. B 103 8799[23] Li X and Coffer J L 1999 Chem. Mater. 11 2326[24] Alivisatos P 2004 Nat. Biotechnol. 22 47[25] Kumar A and Mital S 2001 J. Colloid Interface Sci. 240 459[26] Kumar A and Jakhmola A 2007 Langmuir 23 2915[27] Sun L, Yu C and Irudayaraj J 2007 Anal. Chem. 79 3981[28] Wei G, Zhou H, Liu Z, Song Y, Wang L, Sun L and Li Z 2005

J. Phys. Chem. B 109 8738[29] Kumar A and Kumar V 2007 J. Phys. Chem. C 112 3633[30] Ma N, Dooley C J and Kelley S O 2006 J. Am. Chem. Soc.

128 12598[31] Willner I, Baron R and Willner B 2007 Biosens. Bioelectron.

22 1841[32] Mandal S, Phadtare S and Sastry M 2005 Curr. Appl. Phys.

5 118[33] Graber K C et al 1996 Langmuir 12 2353[34] Colarusso P, Zhang K, Guo B and Bernath P F 1997 Chem.

Phys. Lett. 269 39[35] Kumar A and Jain A K 2003 J. Photochem. Photobiol. A

156 207[36] Ung T, Giersig M, Dunstan D and Mulvaney P 1997 Langmuir

13 1773

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