Revealing the energy transfer between NIR-II PEGylated quantum dots and water

Abstract

Hydrophilic semiconductor quantum dots (QDs) with photoluminescence (PL) located in the second near-infrared window (NIR-II) have shown great promise for in vivo bioimaging applications. However, their performance can be affected by the strong water absorption in this spectral region, which results in severely reduced NIR-II PL intensity. Herein, we use a PEG polymer modified with a hydrophobic alkyl chain (–C₁₁H₂₂–) to decorate non-toxic NIR-II Ag₂S (Ag₂S–C₁₁–PEG) QDs. The incorporation of the alkyl chain minimizes the energy transfer process from QDs to water molecules. As a result, the Ag₂S–C₁₁–PEG QDs exhibit superior PL intensity with approximately three-fold enhancement compared to the Ag₂S QDs coated with PEG without modification (Ag₂S–PEG). Furthermore, the PEG tail in the QDs ensures the biocompatibility of the probe. Ultimately, the Ag₂S–C₁₁–PEG QDs have been demonstrated to have potential as high-performance probes for in vivo bioimaging, exhibiting excellent brightness and metabolizable ability. This work showcases a valid strategy for advancing NIR-II QDs in high-performance bioimaging applications.

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10th anniversary statement

It has been a privilege to witness the growth of Inorganic Chemistry Frontiers over the past decade. My journey with the journal began in 2017, when I first participated in its 2nd ICF Symposium at Nanjing University, and I have since watched it evolve into a leading platform for cutting-edge research in inorganic chemistry. The journal has played a crucial role in fostering global collaboration and pushing the boundaries of scientific discovery. Attending the 10th anniversary symposium in 2024 at Dalian University of Technology was a truly rewarding experience, as it allowed me to reflect on the journal's progress and engage with brilliant minds in the field. The breadth of innovative research showcased reaffirms the journal's commitment to excellence and its pivotal role in shaping the future of inorganic chemistry. I am grateful to be part of this exciting journey and look forward to its more insightful contributions.

Introduction

In vivo fluorescence imaging analysis represents a valuable method in a number of medical contexts, including clinical trials and cancer research.^1–7 Known as the “biological-tissue transparency window”, the second near infrared region (NIR-II) (1000–1700 nm) is an optimal spectral window for bioimaging due to the reduced tissue scattering and autofluorescence noise that is hardly available in other regions.^8,9 Therefore, colloidal semiconductor quantum dots (QDs) with NIR-II emission are widely used in in vivo imaging.^8,10–17 For biological application, the NIR-II QDs are required to disperse in aqueous solvent.^18–24 However, water molecules exhibit strong absorption in the NIR-II region (especially at 1200 and 1400 nm) owing to the bending and stretching vibration of the hydroxyl group (–OH),²⁵ which will lead to photoluminescence (PL) drop of QDs and significant interference of the imaging results.²⁶

Recently, we have discovered that energy transfer (ET) occurs between hydrophilic NIR-II QDs and water when the PL emission of QDs overlaps with the absorption spectrum of water. This phenomenon contributes significantly to the decrease in the PL intensity of hydrophilic NIR-II QDs.²⁷ To address this issue, employing amphiphilic ionic surfactants to create a hydrophobic interface on the QDs has proven to be an effective strategy for inhibiting the ET process. However, the potential biological toxicity and colloidal characteristics of these amphiphilic ionic surfactants pose limitations for their further application in the bioimaging of QD–surfactant systems.

Polyethylene glycol (PEG), as a highly biocompatible hydrophilic polymer, is commonly used for the surface functionalization of nanoparticles in biomedical applications.^28–37 PEGylated QDs with PL emission in the NIR-II window have been extensively applied in bioimaging, but they also experience PL reduction due to the ET process.^38–44 In this study, building on our previous research, we have developed a PEG polymer modified with a hydrophobic alkyl chain (–C₁₁H₂₂–), which serves as a surface ligand to decorate silver sulfide (Ag₂S) QDs with a PL emission peak at 1200 nm (Ag₂S–C₁₁–PEG). The incorporation of the alkyl chain results in a three-fold enhancement in the PL intensity of Ag₂S QDs in aqueous solvents, and significantly prolongs the PL lifetime to a level comparable to that of QDs dispersed in non-aqueous solvents. These findings suggest that the alkyl chain creating the hydrophobic interface effectively suppresses the ET from Ag₂S QDs to water molecules. Finally, the Ag₂S–C₁₁–PEG QDs have demonstrated their potential as high-performance bioimaging probes, exhibiting excellent brightness and good metabolizing ability after intravenous injection into the tails of nude mice. This work showcases a valid strategy to design advancing NIR-II QDs by inhibiting ET for more effective bioimaging applications.

Results and discussion

We first prepared Ag₂S QDs decorated with 1-dodecanethiol (DDT) according to our previous work,² as shown in Fig. S1.† Then, the HS–PEG and HS–C₁₁–PEG ligands (Fig. 1a) were modified on Ag₂S QDs via the ligand exchange method, denoted as Ag₂S–PEG (Fig. 1b) and Ag₂S–C₁₁–PEG (Fig. 1c). Both QDs demonstrated excellent dispersibility in water and chloroform, respectively, as evidenced by the linear relationship between PL intensity and QD concentration (Fig. S2†). Fig. 1d and S3† depict the transmission electron microscopy (TEM) images and corresponding particle size distribution statistics of Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs, indicating their similar morphology, particle size and monodispersity. The optical properties of Ag₂S–PEG and Ag₂S–C₁₁–PEG were tested and recorded at room temperature. In Fig. 1e, the two QDs dispersed in water with the same concentration display almost coinciding absorption curves, suggesting that the different PEG ligands hardly transform the structure of the semiconductor core in the QDs. The PL emission spectra in Fig. 1e show that the two QDs present identical PL emission curves with a peak located at about 1200 nm. The non-Gaussian symmetry of PL spectra is due to the stretching vibration of C–H at 1155 nm of the polymer ligands to absorb the photon emitted by QDs. Interestingly, the Ag₂S–C₁₁–PEG QDs demonstrate a three-fold enhancement in PL intensity compared with Ag₂S–PEG. The absolute photoluminescence quantum yield (PLQY) of Ag₂S–C₁₁–PEG QDs in water is up to 4.8% (Fig. S4†), even higher than those of several hydrophobic Ag₂S QDs obtained by conventional oil-phase synthesis.^12,13 Subsequently, time-resolved PL spectra of both QDs in water are presented in Fig. 1e. According to the fitted lifetime, Ag₂S–C₁₁–PEG QDs possess a longer PL lifetime (81 ns) than Ag₂S–PEG QDs (28 ns), which indicates that the increase of PL originates from the suppression of the non-radiative ET process in the Ag₂S–C₁₁–PEG QDs solution.^45–47

Fig. 1 (a) Structures of SH–PEG and HS–C₁₁–PEG. Schematic illustration of Ag₂S–PEG (b) and Ag₂S–C₁₁–PEG (c) QDs. (d) TEM images of Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs. (e) Room-temperature absorption and PL spectra of Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs dispersed in water. (f) Time-resolved PL spectra of Ag₂S–PEG and Ag₂S–C₁₁–PEG in water.

Then, we investigated the PL emission of the two QDs dispersed in different solvents to further prove the PL enhancement and prolonged PL lifetime originating from the suppression of the ET process. As described in Fig. 2a and b, compared with the Ag₂S–PEG QDs in water, the Ag₂S–PEG QDs in chloroform exhibit about 6.25-fold enhanced PL intensity because of the strong absorption of water molecules at 1200 nm. This was verified by the PL spectrum of Ag₂S–PEG QDs in deuteroxide (D₂O) (Fig. S5†). Since D₂O possesses negligible absorption at 1200 nm, the Ag₂S–PEG QDs in D₂O display higher PL intensity than the QDs in H₂O. The PL decrease of Ag₂S–PEG QDs depends on the overlap of the QD fluorescence spectra and water molecule's absorption (Fig. S6†). The PL drop can also be observed in PbS–PEG QDs. As shown in Fig. S7,† due to strong absorption of water in the 1400–1600 nm range, PEG-capped PbS QDs with emission at 1537 nm when dispersed in water show intense PL quenching and shortened PL lifetime. When utilizing a PEG polymer with alkyl chain modification as a surface ligand to treat Ag₂S QDs, the steady PL spectra of Ag₂S–C₁₁–PEG QDs show that the QDs in chloroform exhibit only a 2.02-times increase in PL intensity compared with that of the QDs in water, and luminescence intensity close to that of Ag₂S–PEG in chloroform (Fig. 2a and b). According to these, the efficiency of ET (η_ET) can be approximately calculated to be 31.3%.

Fig. 2 (a) PL emission spectra, (b) relative PL intensity and (c) time-resolved PL spectra of Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs dispersed in chloroform and water, respectively. Both Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs can be well-dispersed in water or chloroform.

Time-resolved PL spectra in Fig. 2c show that the Ag₂S–C₁₁–PEG QDs give a similar PL lifetime in both water and chloroform, which suggested that Ag₂S–C₁₁–PEG QDs in the two solvents possess the same exciton recombination process, and their PL decrease in water (Fig. 2a) is due to the resonance absorption of –OH.^48,49 As to Ag₂S–PEG, the QDs in water present a much shorter PL lifetime of ∼28 ns than that of QDs in chloroform (86 ns), which indicates the existence of additional nonradiative-exciton-recombination ET channels in Ag₂S–PEG dispersed in water. Based on the chain length of the PEG ligand (Fig. S8†), we think that the ET mechanism in this system is mainly attributed to Förster resonance energy transfer, which occurs at a distance of 2–10 nm.⁵⁰ The mechanism is further corroborated by femtosecond transient absorption (TA) spectra (Fig. S9 and S10†). These indicate that the incorporation of the alkyl chain suppresses the ET process from the Ag₂S core to water molecules. In addition, we have also studied the effect of donor–acceptor distance by utilizing three typical SH–C_n–PEG ligands with different hydrophobic chain lengths coated on Ag₂S QDs (n = 3, 11, 18). As shown in Fig. S11,† we found that Ag₂S–C₁₁–PEG QDs exhibit the highest PL intensity, which may result from the insufficient inhabitation of ET when n = 3 and the low ligand density for the large steric hindrance at n = 18.

In order to explore this process in terms of the energy gained by water, we evaluated the photothermal efficiency (η) of Ag₂S–C₁₁–PEG and Ag₂S–PEG QDs according to the following three equations.⁵¹

τ_s = m_dc_d/(hS) (1)

Q₀ = hS(T_max,water − T_surr) (2)

where τ_s is the time constant of heat transfer; m_d is the unit mass of water (m_d = 1 g), c_d is the heat capacity of water at room temperature [c_d = 4.2 J (g °C)⁻¹], hS is the product of the heat-transfer coefficient and the surface area for radiative heat transfer, Q₀ is the heat dissipated from light absorbed by the quartz sample cell, T_surr is the temperature of surroundings and T_max,water and T_max,sys are the temperatures of water and QD solution in the cell after 600 s exposure with the laser, I is the incident laser power and A₁₀₆₄ is the absorbance of QDs at a 1064 nm wavelength.

Fig. 3a describes the typical plot of temperature versus time obtained from laser irradiation of an optically transparent sample cell filled with Ag₂S–PEG and Ag₂S–C₁₁–PEG at room temperature. A 1064 nm laser with 1 W cm⁻² was used for resonance irradiation. In this experiment, the temperature of Ag₂S–PEG increased from an ambient value of T_surr = 25.1 °C to a maximum equilibrium value of T_max = 48.7 °C after 600 s exposure, while Ag₂S–C₁₁–PEG increased from T_surr = 25.1 °C to T_max = 44.8 °C under the same conditions. Then irradiation with the laser was discontinued and the sample was allowed to cool, returning to a value of T_surr = 25.5 °C in equilibrium with surroundings.

Fig. 3 (a) Temperature of the sample increases during a 600 s continuous irradiation period by a 1 W cm⁻² beam at 1064 nm and returns to its ambient value at thermal equilibrium with the surroundings after discontinuing irradiation. (b) Time constant for heat transfer from the system is determined as τ_s by applying the linearized energy balance to temperature–time data from the cooling period.

To ascertain the efficiency of transducing light to heat of the two QDs, the temperature data were subjected to a linearised energy fitting derived from a description of microscale thermal dynamics in the system. The temperature versus time data recorded during the cooling period as shown in Fig. 3b demonstrate the time constants of heat transfer for Ag₂S–PEG as τ_s = 353.7 s and for Ag₂S–C₁₁–PEG as τ_s = 323.8 s, which represent the negative inverse slope of ln θ with respect to time, denoted as ln θ = −t/τ_s. To circumvent the issue of scatter that arises when the logarithmic operand approaches zero, the data were truncated at 900 seconds. The evaluation of time constants with cooling data permitted the avoidance of effects of thermal gradients during the heating process. By substituting the obtained τ_s into equation eqn (1) and (2), it is possible to obtain hS and Q₀ for the two QDs: Ag₂S–PEG with hS = 0.01187, Q₀ = 0.1086 and Ag₂S–C₁₁–PEG with hS = 0.01297, Q₀ = 0.1167. The photothermal efficiency η of Ag₂S–PEG is 88%, while that of Ag₂S–C₁₁–PEG is 69%. This result demonstrates that the introduction of the C₁₁ chain impedes the occurrence of energy transfer from the perspective of the energy gained by water, and the low energy transfer efficiency consequently leads to low photothermal conversion efficiencies.

Then, we evaluated the performances of the Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs for in vivo NIR-II fluorescence (FL) imaging. Before the in vivo imaging, the biocompatibility of the two QDs was evaluated. A methyl thiazolyl tetrazolium (MTT) assay was executed with mouse fibroblast L929 cells. Fig. S12† illustrates that L929 cells retained their viability under the culture with different concentrations of QDs without statistic variation, revealing their remarkably negligible cytotoxicity because QDs modified with PEG often exhibit distinctive advantages in biocompatibility.^52,53 Subsequently, the well-prepared Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs were injected intravenously into the Balb/c-nude mice at a dosage of 5 mg kg⁻¹, respectively. Then, in vivo NIR-II imaging was executed in real-time to monitor the behavioural differences of the two QDs. The time course of NIR images in Fig. 4a indicates that the Ag₂S–C₁₁–PEG QDs exhibited exceptional capabilities of high-contrast in vivo imaging than Ag₂S–PEG QDs. To visually assess the difference, the quantitative curves of FL intensity are shown in Fig. 4b, and suggest that the NIR-II fluorescence signal of the Ag₂S–C₁₁–PEG QD group was more than three-times higher than that of the Ag₂S–PEG QD group at the peak signal, which is consistent with the brighter Ag₂S–C₁₁–PEG QDs exhibited in Fig. 1e. Subsequently, the NIR-II FL signals gradually decreased with the passage of time due to the metabolism of the QD probes. In addition, we then collected the NIR fluorescence images of the dissected organs after the injection of QDs for 24 h. The results in Fig. 4c, d, and S13† show that NIR fluorescence signals were captured in the liver, stomach and intestine, which also indicated that the two QDs could be metabolized by the hepatobiliary pathway. Encouragingly, obvious NIR fluorescence signals were captured in the faeces of the mice at different time points (Fig. S14†), which further confirmed the above conclusion. Moreover, the stomach exhibited the most intense NIR-II signal, highlighting the high metabolic efficiency of the QDs. Notably, the introduction of an alkyl chain into the PEG structure led to varying distributions of the two QD probes across primary organs. The presence of the hydrophobic group promotes the aggregation of the QDs in the liver. These findings have significant implications for the design of probes tailored for various bio-applications and for advancing our understanding of bio-interface interactions.

Fig. 4 (a) Real-time in vivo NIR-II FL imaging of mice with an intravenous injection of 5 mg kg⁻¹ QDs to monitor the behavioral differences of two QDs of Ag₂S–PEG and Ag₂S–C₁₁–PEG QDs. (b) Quantitative curves of FL intensity in vivo. FL signal distribution of Ag₂S–PEG (c) and Ag₂S–C₁₁–PEG (d) QDs in primary organs.

Conclusions

In summary, we have designed a PEG polymer modified with a hydrophobic alkyl chain (–C₁₁H₂₂–) to decorate NIR-II Ag₂S QDs. As a result, the incorporation of the alkyl chain suppresses the energy transfer process from the QDs to water molecules, leading to a near three-fold enhancement in PL intensity and a prolonged PL lifetime of the QDs. Furthermore, the PEG structure also maintains the biocompatibility of the QD probe. These new modified NIR-II QDs are as well demonstrated with high performance for in vivo bioimaging with excellent brightness and metabolizable ability. This study is of great significance for the design and investigation of the surface chemistry on hydrophilic QDs, thereby facilitating enhanced bioimaging in medical applications.

Author contributions

Conceptualization: Qiangbin Wang and Yejun Zhang. Methodology: Mingzhe Wang, Ziyan Zhang, and Sisi Ling. Investigation: Mingzhe Wang, Ziyan Zhang, and Sisi Ling. Formal analysis: Mingzhe Wang, Ziyan Zhang, and Sisi Ling. Supervision: Hongchao Yang and Qiangbin Wang. Writing – original draft: Mingzhe Wang and Sisi Ling. Writing – review and editing: Mingzhe Wang, Sisi Ling, Yejun Zhang, Hongchao Yang and Qiangbin Wang.

Data availability

The data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22127808 and 22271308), the Strategic Priority Research Program of Chinese Academy of Sciences (CAS) (Grant No. XDB0520301), Key Research Program of Frontier Sciences, and the Natural Science Foundation of Jiangsu Province (Grant No. BK20222016 and 20221262). The authors thank Suzhou NIR-Optics Technologies Co., Ltd for its instrumental and technical support in NIR-II imaging. The authors are grateful for the PL test support from Nano-X from Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (SINANO).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02869d

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