triplet annihilation by gold nanoparticles

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May 12, 2015 - In contrast, sensitized singlet oxygen generation relies on the use of ... fluorescence spectra of RB and DPBF are shown in the ESI.†. Note that ...
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Cite this: Phys. Chem. Chem. Phys., 2015, 17, 14479

Plasmon-enhanced homogeneous and heterogeneous triplet–triplet annihilation by gold nanoparticles†

Received 31st March 2015, Accepted 8th May 2015

Xian Cao, Bo Hu, Rui Ding and Peng Zhang*

DOI: 10.1039/c5cp01876e www.rsc.org/pccp

We report the investigation of surface plasmon induced enhancement of homogeneous and heterogeneous triplet–triplet annihilation (TTA) by gold nanoparticles (AuNPs). Results show that AuNPs enhance the overall efficiency in both cases. Excitation rate and intersystem crossing efficiency of the sensitizer, and efficiency of energy transfer between sensitizer and acceptor are believed to be enhanced by the surface plasmon of AuNPs, leading to the enhancement of overall TTA efficiency.

Interaction between two triplet states of molecules is an important triplet decay pathway, involving both spin and energy exchanges between the pair of molecules or molecular fragments.1,2 It plays an important role in many photophysical processes in chemistry and biology.3–7 Two triplet state molecules often annihilate (upon collision) to produce one molecule in an excited singlet state and another in its ground singlet state. This phenomenon is referred to triplet–triplet annihilation (TTA).8 In principle, the TTA process includes two categories, a homogeneous TTA (Homo-TTA) occurring between two triplet excited molecules of the same type such as the homogeneous TTA upconversion (Homo-TTA-UC), and a heterogeneous TTA (Hetero-TTA) between two triplet excited molecules of different types such as the heterogeneous TTA upconversion (Hetero-TTA-UC),9 or between a triplet excited state and a triplet ground state such as the sensitized singlet oxygen generation.10 TTA upconversion (TTA-UC) has drawn increasing attention in recent years, largely because it can occur with low-intensity, non-coherent light as the excitation sources.11 TTA-UC process involves energy transfer (ET) from excited sensitizer molecules to triplet-excited acceptor molecules, which subsequently undergo annihilation and populate the singlet-excited state of the acceptor. Emission from the singlet-excited acceptors has a shorter wavelength than the initial excitation of the sensitizers,

Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c5cp01876e

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thus achieving the upconversion. To date, significant progress has been made for TTA-UC with varying degrees of success, ranging from exploring different combinations of sensitizers and acceptors, and in different media including various solvents and polymers.12–16 In contrast, sensitized singlet oxygen generation relies on the use of sensitizers in the presence of light and oxygen.17,18 The sensitizer molecule absorbs light of appropriate wavelengths to reach the singlet-excited state, followed by an intersystem crossing to reach its triplet state. Singlet oxygen is generated via an energy transfer accompanied with the collision between the triplet-excited sensitizer and the ground state oxygen, a triplet.7 Thus sensitized singlet oxygen generation is in essence a heterogeneous TTA process.19 Singlet oxygen plays a critical role in photodynamic therapy against both cancers and bacteria.20 There have been a lot of efforts in identifying, designing and synthesizing molecules as efficient sensitizers.7,18 Still, there is ongoing need to explore ways to enhance the efficiency of singlet oxygen generation, especially under red and near-infrared excitations. Noble metal nanostructures are known to exhibit an extraordinary capability to manipulate light through the collective oscillations of their conduction-band electrons, the so-called localized surface plasmon resonances (LSPR).21 Plasmonic nanostructures are shown to be able to dramatically enhance the performances of many optical devices.21 Previous investigations have revealed the enhancement of photoluminescence through LSPR.22,23 Recent efforts also demonstrated approaches utilizing the broad surface plasmon of silver island film to enhance the excitation and singlet and triplet emissions of the sensitizers, resulting in both metal-enhanced fluorescence and phosphorescence.24,25 Another study reported metal-enhanced TTA-UC through surface plasmon resonance of thin silver film, although the inhomogeneous nature of the film rendered it difficult to quantitatively assess the enhancement in upconversion efficiency.26 Herein we report the investigation of plasmon-enhanced homogeneous and heterogeneous TTA processes by gold nanoparticles (AuNPs). Rose Bengal (RB) is covalently conjugated to AuNPs,

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and serves as the sensitizer ([email protected]) in both cases, while 1,3-diphenylisobenzofuran (DPBF) as the acceptor in TTA-UC. The [email protected]–DPBF system displays 1.6-fold enhancement of TTA upconversion compared to its counterpart without AuNPs (RB–DPBF). A 13-fold enhancement of singlet oxygen generation was observed with [email protected] under Xenon lamp irradiation compared to free RB. The molecular structures and normalized absorption and fluorescence spectra of RB and DPBF are shown in the ESI.† Note that RB has a strong absorption peak at B560 nm and a fluorescent peak centered at B585 nm in N,N-dimethylformamide (DMF), while DPBF does not absorb in the spectral region where RB emits and emits at B485 nm, shorter than the absorption wavelength of RB. Considering that TTA process is a Dexter-type energy transfer process based on molecular collisions between the triplet acceptors, high mobility of acceptor is desired. Therefore, we conjugate AuNPs to RB (sensitizer), rather than to DPBF (acceptor), through the widely used EDC/sulfo-NHS chemistry. The resulting [email protected] hybrid nanoparticles are also used as sensitizers to produce singlet oxygen in the presence of oxygen under light irradiation of appropriate wavelengths. Cysteamine-capped AuNPs were synthesized according to the previously reported procedure (ESI†).27 The as-prepared AuNPs have positive charges and amine groups on the surface, which can be used to conjugate to the carboxylic group of RB through the EDC/sulfo-NHS method. TEM image of AuNPs after conjugation with RB, and normalized absorption spectra of RB, AuNPs and [email protected] are shown in Fig. S1 (ESI†). The absorption spectrum of [email protected] includes the typical surface plasmon peak of AuNPs at B520 nm and the pure RB absorption peak at B560 nm, indicating the successful conjugation of AuNPs and RB. Normalized absorption and emission spectra of DPBF and RB are shown in Fig. S3 (ESI†). The concentration (0.11 mM) of RB in [email protected] was determined by dissolving the Au component of a known amount of [email protected] using NaCN and comparing the absorbance of the remaining RB to a calibration curve of pure RB solutions (Fig. S4 in ESI†). We first investigate the case of homogeneous TTA between RB as the sensitizer and DPBF as the acceptor. Fig. 1 illustrates the [email protected]–DPBF upconversion system. DPBF was first dissolved into N,N-dimethylformamide (DMF), then mixed with the [email protected] solution. The mixture was deoxygenated through

Fig. 1 Schematic illustration of plasmon-enhanced TTA-UC of [email protected]– DPBF system.

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freeze–pump–thaw cycle for 3 times before measured. Upconversion spectral measurements were carried out on a homebuilt setup as described previously9 and illustrated in Fig. S2 (ESI†). All samples were excited by a 532 nm-laser. In parallel, an equal volume of [email protected] solution was treated by an excess amount of NaCN to dissolve the AuNPs and release RB. The resulting solution was mixed with DPBF solution, and also treated by three freeze–pump–thaw cycles before spectrum was taken. Comparison of the spectra with and without the NaCN treatment allows us to determine the effect of AuNPs on the TTA-UC efficiency. Fig. 2a and b show the upconversion and fluorescence spectra of the [email protected]–DPBF mixture with and without the NaCN treatment. Separately, a control experiment using the mixture of NaCN and RB–DPBF was performed to verify that NaCN itself has no effect on the TTA-UC of RB–DPBF mixture. It is observed that the upconversion intensity from [email protected]–DPBF is higher than that from RB–DPBF, while the fluorescence intensity centered at 585 nm of RB from the [email protected]–DPBF is lower than that from RB–DPBF. To confirm the observation of metal-enhanced TTA-UC, the overall quantum yield (QY) of the TTA-UC was determined following procedures described previously9 and included in Fig. S5 (ESI†). The QY of TTA-UC as a function of the DPBF concentration is shown in Fig. 2c. As expected, the TTA-UC QYs from [email protected]–DPBF with different DPBF concentrations increased in the presence of AuNPs. The relative enhancement factor (EF), defined as the ratio of the TTA-UC QY of [email protected]RB–DPBF over that of RB–DPBF, is used to quantify the metal enhancement of TTA-UC. Fig. 2d shows the EF as a function of DPBF concentration. The results illustrate that EF values are fairly consistent (B1.6) regardless of the DPBF concentration except for the first point (B1.2), of which the upconversion is rather weak due to the low DPBF concentration. These values are lower than what

Fig. 2 (a, b) Upconversion and fluorescence spectra of RB (10.8 mM)–DPBF (1.6 mM) and [email protected] (10.8 mM)–DPBF (1.6 mM) mixtures in DMF solution, excited by a 532 nm laser. Power density: 40 mW cm 2. (c) Quantum yield of RB (10.8 mM)–DPBF and [email protected] (10.8 mM)–DPBF systems as a function of [DPBF], respectively. (d) Enhancement factor as a function of [DPBF].

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was recently reported in a TTA-UC system (B6.6) involving physical mixing of sensitizers, acceptors and silver nanoplates in a solid polymer matrix.28 Since all of our measurements were carried out in DMF solution, we consider these results to be more consistent, not susceptible to uneven signals caused by the geometry of the solid samples. It has been well understood that localized plasmonic enhancement can be orders of magnitude higher than the average enhancement for the whole structure.29 The EF values in this setting represent the average, instead of the localized, metal enhancement effect. The schematic of TTA-UC when RB is bound to AuNPs is illustrated in Fig. 3. The overall quantum yield of TTA-UC for the [email protected]–DPBF system FUC can be described in eqn (1) as: FUC = gexFISCFETFTTAFFecoll

(1)

where gex, FISC, FET, FTTA, FF and ecoll are the excitation rate of the sensitizer, intersystem crossing efficiency of the sensitizer, energy transfer efficiency between sensitizer and acceptor, triplet–triplet annihilation efficiency of the acceptor, fluorescence quantum yield of the acceptor, and the light collection efficiency of the instrument. In eqn (1), gex, FISC, FET are associated with the sensitizer, while FTTA and FF are associated with the acceptor only. Thus it is expected that metal enhancement, if existing, would reflect in the first three terms, since the sensitizer is covalently bound to the AuNPs. We consider that it is critical to have significant overlap between the sensitizer absorption and the AuNPs plasmon band in order to observe the metal enhancement effect. In the [email protected]–DPBF system, the RB absorption peak wavelength is B560 nm, matching well with the plasmon band of AuNPs in Fig. S1b (ESI†). For comparison, we conjugated RB to silver nanoparticles, which have a plasmon band centering at B420 nm, and paired with DPBF for upconversion measurements. As shown in Fig. S6 (ESI†), the resulting [email protected]– DPBF system shows little difference from the corresponding RB–DPBF system, i.e., AgNPs do not enhance the TTA-UC of RB–DPBF. This result supports the notion that metal enhancement of TTA-UC is attributed to the plasmon-sensitizer resonance coupling between AuNPs and RB. All three terms, gex, FISC, FET, in the [email protected] complex can be enhanced by the plasmon-sensitizer resonance coupling. The excitation of the AuNPs plasmon can greatly enhance the local electromagnetic field on the AuNPs surface, leading to the increase of gex. FISC could be increased, as the highly enhanced

Fig. 3

Illustration of plasmon-enhanced TTA-UC by AuNPs.

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and inhomogeneous field on the AuNPs surface could promote the mixing of electronic states of opposite parities by the Stark effect,30 and enhance the spin–orbit couplings between the singlet–triplet spin of the molecules. This is supported by the observation of lower fluorescence intensity (centered at 585 nm) of [email protected]–DPBF, as compared to that of RB–DPBF, as shown in Fig. S7a (ESI†), suggesting the increase of FISC. The energy transfer (FET) between the triplet states of sensitizer and acceptor, which is of Dexter-type, could be increased as well because the enhanced localized electromagnetic field can promote the electron exchange. To sum up, it is likely that three terms in the TTA-UC processes, gex, FISC, and FET, are enhanced by the presence of AuNPs in the vicinity of RB, as indicated by the purple lines in Fig. 3. In the current design where RB is conjugated to AuNPs, the metal effect on FTTA and FF are probably negligible. It would be interesting to construct a TTA-UC sensitizer-acceptor pair where the acceptor is bound to the metal nanoparticle, and study the metal effect on those terms. To further investigate the plasmon-enhanced TTA-UC, time decay of the TTA-UC emission was measured. Fig. 4 shows the normalized intensity decay curves of upconversion emission at 485 nm from [email protected]–DPBF and RB–DPBF, respectively. All of the decay curves can be fitted well by a single-exponential decay function (details in Fig. S8 of ESI†). Notice that the decay time of the upconversion emission of [email protected]–DPBF is longer than that of RB–DPBF. A longer decay time of upconversion from [email protected]–DPBF indicates that the triplet sensitizer has longer lifetime, which would promote energy transfer between the triplets of sensitizer and acceptor (TTET) to occur. These results are in line with the results of the steady-state measurements and provide strong evidence of plasmon-enhanced TTA-UC in [email protected]–DPBF system. We next consider the case of heterogeneous TTA between RB as the sensitizer and ground state oxygen as the acceptor, leading to the sensitized generation of singlet oxygen. Singlet oxygen emission spectra were collected on a spectrofluorometer excited by a Xenon lamp as described in ESI.† Three different concentrations of sensitizers were used to investigate the effect of AuNPs on singlet oxygen generation. Fig. 5a–c show the singlet oxygen emission spectra from [email protected] and RB after removing AuNPs from [email protected] Clearly, the singlet oxygen emission intensity increases with the increased concentration of sensitizer, and the intensities

Fig. 4 Normalized intensity decay curves of TTA-UC emission at 485 nm of [email protected] (10.8 mM)–DPBF (2 mM) and RB (10.8 mM)/DPBF (2 mM) in deoxygenated DMF solutions.

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Fig. 6 Illustration of plasmon-enhanced sensitized singlet oxygen generation.

Fig. 5 Singlet oxygen emission spectra of [email protected] and RB in DI water, excited at 540 nm. RB concentration is: (a) 24.2 mM, (b) 9.5 mM (c) 0.7 mM. (d) EF of singlet oxygen generation of [email protected] as a function of RB concentration.

from [email protected] are much higher than the one from the corresponding RB. Singlet oxygen enhancement factor (EF) is determined by EF = Ih/Ip, where Ih and Ip are the singlet oxygen emission intensity of [email protected] and RB, respectively. Results shown in Fig. 5d confirm that there is significant enhancement in singlet oxygen generation of [email protected] as compared to RB. Similar to the case of homogeneous TTA-UC, the observed singlet oxygen emission intensity of [email protected] complex can be described in eqn (2) as: I = gexFISCFETFPecoll

(2)

where gex, FISC, FET, FP and ecoll are the excitation rate of sensitizer, intersystem crossing efficiency of sensitizer, efficiency of energy transfer from triplet sensitizer to ground state oxygen, singlet oxygen emission efficiency, and light collection efficiency of the instrument, respectively. The plasmon peak of AuNPs is B520 nm, far away from singlet oxygen emission peak at B1270 nm. Thus there should be little resonance between the surface plasmon of AuNPs and singlet oxygen emission.31 Therefore the terms in eqn (2) that would be enhanced by AuNPs in the [email protected] complex are gex, FISC, and FET, which is similar to the scenario of homogeneous TTA-UC. The increase of FISC is also reflected in the reduced fluorescence lifetime of RB in [email protected], as compared to the free RB (Fig. S9, ESI†). Accordingly, a diagram of plasmon-enhanced singlet oxygen generation by AuNPs is illustrated in Fig. 6, where the red lines indicate the terms affected by AuNPs. We shall point out that, surface plasmon resonance between the metal nanostructures and the absorption of sensitizers is critical to the enhancement of singlet oxygen generation. While it is possible to design and construct metal nanostructures that display surface plasmon resonance with the singlet oxygen emission, such effort would only lead to the improved detection, but not the generation, of singlet oxygen. In summary, we report a thorough investigation on the enhancement of homogeneous and heterogeneous TTA by surface

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plasmon induced by AuNPs. The AuNPs are conjugated to RB, the sensitizer in both types of TTA process. Results show that the HomoTTA-UC efficiency and singlet oxygen generation efficiency are both enhanced by AuNPs, by up to 1.6-fold and 13-fold, respectively. Excitation rate and intersystem crossing efficiency of the sensitizer, and efficiency of energy transfer between sensitizer and acceptor are believed to be enhanced by the surface plasmon of AuNPs, leading to the enhancement of overall TTA efficiency. The results shed light onto ways to improve the overall TTA efficiency, which would be of use to the broad applications involving TTA upconversion or singlet oxygen generation. Partial support from US DOD Award DM102420 (W81XWH11-2-0103) is gratefully acknowledged.

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