Investigation of the mechanism and dynamics of resonance excitation energy transfer from photoexcited donors to metal nanoparticles and nanoclusters

Abstract

The resonance excitation energy transfer (EET) from various photoexcited donors to acceptors has been studied thoroughly in the recent past due to its importance in photovoltaics, light-emitting diodes, sensors, bioimaging, etc. Nonradiative energy transfer from a photoexcited donor to an acceptor via dipole-dipole interactions is well known as Förster resonance energy transfer (FRET). Over the last few years, metal and semiconductor nanoparticle (NPs) based donor-acceptor composite systems have gained considerable attention due to their size-dependent optoelectronic properties which allow easy tuning of energy transfer efficiency. For example, semiconductor NPs or quantum dots have been extensively used as FRET-based donor due to their stable and bright sizedependent photoluminescence (PL) in the visible region of the electromagnetic spectrum. It has been observed that metal NP with distinct localized surface plasmon resonance (LSPR) often quenches the molecular excitation energy of nearby fluorophore by nanometal surface energy transfer (NSET), which is quite different from the conventional FRET. Extensive theoretical and experimental studies have been performed to understand the mechanism behind the highly efficient quenching of molecular excitation energy of various photoexcited fluorophores by metal NPs. Some reports have illustrated the PL quenching of photoexcited donors in the presence of metal NPs by FRET mechanism, whereas several other studies have demonstrated that EET from photoexcited donor to metal NP is best modeled by NSET theory. Moreover, it has been reported that NSET does not require the spectral overlap between the emission spectrum of donor and absorption spectrum of acceptor. In contrast, several researchers have validated the involvement of spectral overlap in NSET process. Despite numerous reports, the fundamental mechanism of EET from various photoexcited donors to metal NP as well as the role of spectral overlap in the EET process still remains obscure.Moreover, very less is known about the mechanism and dynamics of fluorescence quenching near the ultrasmall metal nanocluster (NC) surface, which lacks characteristic LSPR. In this thesis, the detailed mechanism and dynamics of EET from various photoexcited donors such as silicon quantum dots (Si QDs), carbon dots (CDs), and 4´,6-diamidino- 2-phenylindole (DAPI) to silver nanoparticles and nanoclusters (Ag NP and Ag NC) as acceptors have been demonstrated. The role of spectral overlap between donor emission spectrum and LSPR band of Ag NPs in NSET process as well as the effect of different sizes of NPs on NSET efficiency has been clearly presented. Furthermore, the influence of various microheterogeneous environments such as surfactants, polymer, and DNA on the efficiency of energy transfer has been illustrated. The contents of each chapter included in the thesis are discussed briefly as follows:1. Introduction In this chapter, a brief overview of the unique optical properties of Ag NP, Ag NC, Si QD, and CD has been provided. The effect of metal NPs and NCs on the emission properties of various photoexcited fluorophores has been briefly explained. The mechanism and dynamics of various EET theories have been discussed in detail. Further, the importance of spectroscopic nanorulers such as FRET and NSET, in numerous biologically relevant microheterogeneous environments has been illustrated. 2. Materials and experimental techniques The details of all the chemicals and complete synthetic procedures of citrate-stabilized Ag NP, dihydrolipoic acid-capped Ag NC, allylaminefunctionalized Si QD, and CD have been mentioned here. A brief description of the sample preparations and experimental techniques used to complete the entire work of this thesis has been provided.3. Surfactant-induced modulation of nanometal surface energy transfer from silicon quantum dots to silver nanoparticles In this chapter, the EET from Si QD to Ag NP and its modulation in the presence of CTAB surfactant has been demonstrated by means of steady-state and time-resolved PL spectroscopy. Significant spectral overlap between the emission spectrum of Si QDs and localized surface plasmon resonance (LSPR) of Ag NPs results in a substantial amount of PL quenching of Si QDs. In addition, the PL lifetime of Si QDs is shortened in the presence of Ag NPs. The origin of this PL quenching has been rationalized on the basis of increased nonradiative decay rate due to EET from Si QDs to Ag NPs surface. The observed energy transfer efficiency correlates well with the NSET theory with 1/d4 distance dependence rather than conventional FRET theory. It has also been observed that the EET efficiency drastically reduces in the presence of 0.5 mM CTAB. Dynamic light scattering (DLS), and single particle PL microscopy results indicate the formation of large surfactant-induced aggregates of Ag NPs. Finally, the energy transfer efficiency values obtained from experiment have been used to calculate the distance between Si QDs and Ag NPs in the absence and presence of CTAB, which correlates well with the proposed model.4. Resonant excitation energy transfer from carbon dots to different sized silver nanoparticles The influence of size on the efficiency of NSET process between excited donor and different sized metal NP is poorly explored in literature. Here in this work, a systematic study has been demonstrated by correlating the size of Ag NPs with the efficiency of EET from photoexcited CD to Ag NP. Three different sized citrate-capped Ag NPs with mean hydrodynamic diameter of 39.91 ± 1.03, 53.12 ± 0.31 and 61.84 ± 0.77 nm have been synthesized for the present study. The estimated zeta potential of synthesized CD is -25.45 ± 1.23 mV while that for the smallest,medium and largest sized Ag NP is -76.24 ± 3.92, -67.60 ± 4.40, and - 58.01 ± 3.10 mV. It has been observed that the spectral overlap between the emission spectrum of CD and LSPR band of Ag NP increases with increase in the size of Ag NPs. The steady-state and time-resolved PL measurements reveal significant PL quenching of CD as a function of Ag NP size. A control experiment with Ag NPs having LSPR at 398 nm shows negligible amount of PL quenching of CDs. The negligible PL quenching of CD in the presence of Ag NP having inadequate spectral overlap confirms the resonant EET from photoexcited CD to different sized Ag NPs. The separation distances between CD and different sized Ag NPs estimated using FRET theory exceed the FRET limit. The calculated EET related parameters correlate well with the 1/d4 distancedependent NSET theory. Further, it has been observed that the NSET efficiency increases with increase in the size of Ag NPs. This phenomenon has been explained by considering larger spectral overlap and shorter separation distance between CD and larger sized Ag NPs due to reduced electrostatic repulsion. These results reveal that the size of NP plays an important role in the NSET process and this phenomenon can be easily utilized to tune the efficiency of energy transfer for various applications. 5. Effect of compartmentalization of donor and acceptor on the ultrafast resonance energy transfer from DAPI to silver nanoclusters In this chapter, the mechanism and dynamics of EET from photoexcited DAPI to dihydrolipoic acid-capped Ag NC and its subsequent modulation in the presence of cationic polymer PDADMAC and CT-DNA have been demonstrated using steady-state FL and femtosecond FL upconversion techniques. The synthesized Ag NCs were characterized using FTIR spectroscopy, mass spectrometry, XPS, HRTEM, DLS, UV-Vis and PL spectroscopy. The mass spectrometric analysis reveals the formation of ultrasmall Ag4 NCs with a small amount of Ag5 NCs. UV-Vis and PL spectra show distinct molecular-likeoptoelectronic behavior of these ultrasmall Ag NCs. The Ag NCs strongly quench the FL of DAPI with concomitant increase in its PL intensity at 675 nm. This steady-state FL quenching proceeds with a significant shortening of FL lifetime of DAPI in the presence of Ag NCs, signifying the nonradiative FRET from DAPI to Ag NCs. Various energy transfer related parameters have been estimated from FRET theory. The present FRET pair shows a characteristic Förster distance of 2.45 nm and can be utilized as a reporter of short-range distances in various FRET-based applications. Moreover, this nonradiative FRET completely suppresses in the presence of both 0.2 wt% PDADMAC and CT-DNA. Steady-state PL, HRTEM, and PL imaging measurements reveal efficient and complete encapsulation of acceptor (Ag NCs) within the polymer matrix. It has been observed that the nonradiative FRET process completely suppresses in the presence of CT-DNA due to the selective binding of DAPI with CT-DNA. This selective compartmentalization of donor and acceptor and the subsequent modification of FRET process may find application in various sensing, photovoltaic, and light harvesting applications. 6. Effect of surfactant assemblies on the resonance energy transfer from 4',6-diamidino-2-phenylindole to silver nanoclusters Here, the effect of SDS assemblies on the FRET between DAPI and Ag NC in phosphate buffer has been demonstrated by using FL spectroscopy. While DAPI interacts specifically with SDS surfactants in a concentration-dependent manner, Ag NC shows no specific interaction with surfactant assemblies. At very low concentrations of SDS (< 0.6 mM), DAPI forms surfactant-induced aggregates at the interface. In the case of intermediate SDS concentrations (0.6 mM ≤ SDS < 4 mM), DAPI associates with the negatively charged SDS pre-micelles via electrostatic interaction. Beyond 4 mM SDS, the FL intensity of DAPI saturates due to complete incorporation of DAPI into the micellar Stern layer. The negligible changes in the FL of DAPI upon addition of non-ionic triton X100 (TX-100) and cationic CTAB surfactants indicate minimal interaction of DAPI with TX-100 and CTAB. Hence, the significant FL enhancement of DAPI in the presence of SDS is due to the specific electrostatic interactions between the positively charged DAPI and negatively charged SDS. Notably, the interaction between DAPI and Ag NC significantly perturbs in the presence of SDS. In phosphate buffer, FRET efficiency (ϕEff) of 78% has been estimated for DAPI-Ag NC pair. This ϕEff decreases to 23% in the presence of 1 mM SDS. Furthermore, in the presence of 16 mM SDS, complete suppression of this nonradiative FRET has been observed due to incorporation of DAPI into the micellar Stern layer. 7. Conclusion and future scope The conclusions of the entire research work of this thesis described here are as follows: 1) The observed PL quenching of Si QDs in the presence of Ag NPs correlates well with NSET rather than FRET model. The NSET efficiency significantly reduces in the presence of 0.5mM CTAB. The CTAB molecules not only induce aggregation of Ag NPs, but also provide an extra bilayer shell on top of the citrate-capped Ag NPs, due to which the mean separation distance between Si QD and the surface of Ag NP increases. As a result, the NSET efficiency decreases. 2) The steady-state and time-resolved lifetime measurements reveal that the observed PL quenching of CD in the presence of different sized Ag NPs is due to the resonant EET. The separation distances between CD and different sized Ag NPs estimated using FRET theory exceed the FRET limit. The calculated EET related parameters correlate well with NSET theory. Moreover, the NSET efficiency increases with increase in the size of Ag NPs. It was observed that with increase in the size of Ag NPs, the spectraloverlap increases. More importantly, the zeta potential of Ag NPs decreases with increase in the size and as a consequence, the effective distance between CD and Ag NPs decreases due to reduced electrostatic repulsion. Hence, these results reveal that the efficiency of NSET process can be easily tuned as a function of NP size. 3) The significant FL quenching of DAPI by Ag NC with a concomitant increase in the PL of Ag NCs clearly signifies the involvement of FRET from photoexcited DAPI to Ag NC. Selective compartmentalization of Ag NC and DAPI has been observed in the presence of 0.2 wt% PDADMAC and CT-DNA, respectively. Moreover, complete suppression of the nonradiative FRET has been observed in the presence of both 0.2 wt% PDADMAC and CT-DNA. 4) It has been shown that the specific electrostatic interactions of DAPI with SDS significantly alter the FRET process between DAPI and Ag NC. The FRET efficiencyof DAPI-Ag NC pair reduces from a bulk value of 78% to 23% in the presence of 1 mM SDS. Furthermore, this FRET completely suppresses in the presence of 16 mM SDS due to the incorporation of DAPI into the micellar Stern layer of SDS. Further, the relevant future scope of the work described in this thesis has been discussed briefly.