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Highly luminescent water-dispersible NIR-emitting wurtzite CuInS2/ZnS core/shell colloidal quantum dots Chenghui Xia, Johannes D. Meeldijk, Hans C Gerritsen, and Celso de Mello Donegá Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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Chemistry of Materials

Highly luminescent water-dispersible NIRemitting wurtzite CuInS2/ZnS core/shell colloidal quantum dots Chenghui Xia,a,b Johannes D. Meeldijk,c Hans C. Gerritsen,b and Celso de Mello Donegaa* a. Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands b. Molecular Biophysics, Debye Institute for Nanomaterials Science, Utrecht University, 3508 TA Utrecht, Netherlands. c. Electron Microscopy Utrecht, Debye Institute for Nanomaterials Science, Utrecht University, 3584 CH Utrecht, Netherlands.

*Corresponding author: E-mail: [email protected]

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ABSTRACT. Copper indium sulfide (CIS) QDs are attractive as labels for biomedical imaging since they have large absorption coefficients across a broad spectral range, size- and composition-tunable photoluminescence from the visible to the near-infrared, and low toxicity. However, the application of NIR-emitting CIS QDs is still hindered by large size and shape dispersions and low photoluminescence quantum yields (PLQYs). In this work, we develop an efficient pathway to synthesize highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from template Cu2-xS nanocrystals (NCs), which are converted by topotactic partial Cu+ for In3+ exchange into CIS NCs. These NCs are subsequently used as cores for the overgrowth of ZnS shells (≤ 1 nm thick). The CIS/ZnS core/shell QDs exhibit PL tunability from the first- to the second NIR window (750 - 1100 nm), with PLQYs ranging from 75% (at 820 nm) to 25% (at 1050 nm), and can be readily transferred to water upon exchange of the native ligands for mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS QDs possess good colloidal stability over at least 6 months and PLQYs ranging from 39% (at 820 nm) to 6% (at 1050 nm). These PLQYs are superior to those of commonly available water-soluble NIR-fluorophores (dyes and QDs), making the hydrophilic CIS/ZnS QDs developed in this work promising candidates for further application as NIR-emitters in bioimaging. The hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shelling are also attractive as fluorophores in luminescent solar concentrators.


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INTRODUCTION

Colloidal semiconductor quantum dots (QDs) have attracted much attention as luminescent probes for bioimaging due to their outstanding optical properties, such as broad absorption spectra, narrow photoluminescence (PL), large absorption cross-sections, high PL quantum yields (PLQYs), and high photostability, which make them superior to organic dyes and fluorescent proteins.1 Moreover, their PL can be tuned throughout the visible to the near-infrared (NIR) spectral window by controlling their composition and size.1 These properties have been translated into higher sensitivity, multiplexed detection (i.e., multiple PL colors upon single excitation wavelength), and longer observation times.1 Colloidal QDs can also be used as multimodal probes, allowing two or more imaging techniques (e.g., MRI and optical) to be combined.2 NIRemitting QDs are of particular interest, since wavelengths in the first and second biological spectral windows (viz. 650-950 nm and 1000-1350 nm, respectively)3 penetrate much deeper in tissue than visible light, while inducing negligible autofluorescence.3-5 However, most currently used NIR-emitting QDs contain highly toxic elements such as Cd, Pb, and As (e.g., CdTe,6-8 CdSe/CdTe,9 Cd3P2,10 InAs,11, 12 PbS,13-15 PbSe16), which severely hinders their application as biolabels. The search for less toxic alternatives is therefore becoming an increasingly relevant topic. Amongst the alternatives, copper indium sulfide (CIS) QDs are particularly promising, since they combine low toxicity17-19 with large absorption coefficients across a broad spectral range and unparalleled PL tunability, spanning a spectral window that covers the PL tunability of CdSe (visible), InP and CdTe/CdSe (visible and first NIR biological window), and PbS (second NIR biological window).20 Nevertheless, to date high PLQYs (≥50%) have only been reported for CIS/ZnS and CIS/CdS core/shell QDs with core diameters below 4 nm and PL up to 750 nm.17-19, 3 ACS Paragon Plus Environment


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Although luminescent CIS QDs larger than 4 nm have also been reported,21, 25-27 their size

and shape dispersion is typically quite large due to the difficulty in balancing the reactivities of multiple precursors (In, Cu, S).20 Partial topotactic Cu+ for In3+ cation exchange (CE) in template Cu2-xS nanocrystals (NCs) has been recently established as an effective strategy to circumvent these limitations,28, 29 allowing the preparation of monodisperse luminescent CIS QDs and NCs of sizes and shapes that would not be easily attainable by direct routes. Interestingly, CIS QDs and NCs obtained by CE adopt the hexagonal wurtzite (WZ) structure, instead of the cubic chalcopyrite (CP) structure typically observed for CIS QDs synthesized by direct routes.28, 29 This creates new opportunities to expand the spectral tunability of CIS QDs, since WZ CIS QDs emit at lower energies than their CP counterparts.30 The PLQY of bare CIS QDs is however low (8), so as to deprotonate the thiol group, thereby ensuring a strong bond between the thiolate head-group and the surface of the CIS/ZnS QDs. However, the high pH will also give rise to the formation of disulfide bonds that may trap the photogenerated hole, resulting in PL quenching.49 To prevent disulfide formation, reducing agents (e.g., sodium borohydride,51 dithiothreitol,52 TCEP49) are commonly added during the phase transfer. In this work, tris-(2carboxyethyl)phosphine hydrochloride (TCEP) was chosen as the reducing agent due to its high efficiency and low contamination during biolabeling.53 The phase transfer was accomplished at pH ~11.6, which resulted in a high concentration of deprotonated MUA thiolate groups that exchanged the native ligands at the surface of CIS/ZnS core/shell QDs, thereby making them hydrophilic (Figure 5). The average Zeta potential of these water-dispersible CIS/ZnS core/shell QDs is about -60 mV (SI Figure S20). The negatively charged surface is attributed to the deprotonation of the carboxyl groups of MUA. This assignment is confirmed by a weak stretching mode of the COOH group at 1720 cm-1 and the appearance of a new band at 1398 cm-1, which is related to the symmetric vibration modes of the COO- group (SI Figure S20).



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Figure 5. Schematic illustration of the phase transfer of CIS/ZnS core/shell QDs by ligand exchange using MUA in the presence of TCEP at pH~11.6. The optical properties and morphology of ~5 nm diameter WZ CIS/ZnS core/shell QDs before and after phase-transfer to water are shown in Figure 6. The average size and polydispersity of the QDs are not significantly affected by the phase-transfer procedure (Figure 6b), and the resulting negatively-charged QDs are colloidally stable in water for long periods of time (at least 6 months). The hydrodynamic size of the CIS/ZnS core/shell QDs is 14.2 ± 2.7 nm (SI Figure S20). The absorption spectra of the QDs before and after the transfer are indistinguishable (figure 6a), indicating that they remain intact and that no aggregates are formed, which is also clearly demonstrated by the TEM images (Figure 6b and SI, Figure S21). The PLQY decreases from 75% to 39% after phase-transfer (Figure 6). The PL spectrum also remains essentially the same, apart from a small blue-shift (19 meV) from 820 nm to 810 nm. The origin of this blue-shift is not yet understood, but may be related to the effect of the negative surface charges on the amplitude of


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the electron wavefunction near the QD surface (assuming that the hole is localized within the core, as discussed above).

Figure 6. (a) Absorption and PL spectra of ∼5 nm CIS/ZnS core/shell QDs before and after phase transfer into water. (b) TEM images and size histograms of CIS/ZnS core/shell QDs before (left) and after (right) ligand exchange (larger area TEM images are given in the SI, Figure S21).

A decrease in PLQY from 26% to 6% upon phase-transfer to water was also observed for 9.2 nm CIS/ZnS core/shell QDs emitting at 1050 nm (SI Figure S22). Reductions in PLQY upon transfer of core/shell QDs to water are commonly observed,17,18,54 and are attributed to insufficient shell quality in part of the QD ensemble.54 In the present case, the drop in the PLQY of the CIS/ZnS core/shell QDs in water may be interpreted as an indication that the ZnS shell in some of the QDs is not yet sufficiently robust to withstand the high pHs required for the ligand exchange. Therefore, it is likely that further improvements in the ZnS shell thickness and quality will lead to higher PLQYs after the phase transfer. Nevertheless, it should be noted that the PLQYs observed in the present work (viz. 39% at 810 nm and 6% at 1030 nm) are rather high for 29 ACS Paragon Plus Environment


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NIR-emitters in water, and are very competitive with respect to the currently available NIRemitting QDs and dyes (CIS/ZnS QDs: 20% at 800 nm,18 8% at 830 nm,17 Indocyanine green: 12% at 835 nm in DMSO,35 IR-140: 17% at 830 nm,55 ONITCP: 2.3% at 900 nm,55 ODNITCP: 1.4% at 930 nm55). It is worth noting that the stability of the hydrophilic CIS/ZnS core/shell QDs in water is also competitive, since they can be stored in the dark at 4 ˚C for at least 6 months with negligible PL loss. Moreover, these QDs are carboxyl terminated and can thus be easily functionalized with specific proteins or biomacromolecules,8,12,17,18,51,56-58 what makes them promising as NIR-emitting labels for bioimaging.

CONCLUSIONS In conclusion, we have developed a sequential procedure to synthesize highly luminescent waterdispersible NIR-emitting wurtzite CIS/ZnS core/shell QDs. The procedure starts from nearly spherical template Cu2-xS NCs, which are converted into wurtzite CIS QDs with size and shape preservation by topotactic partial Cu+ for In3+ cation exchange at 125 °C. The use of a nearly stoichiometric TOP/In ratio (0.9) ensures that the Cu+-extraction and In3+-incorporation rates remain coupled despite relatively high reaction temperatures, leading to a highly versatile and fast method to obtain CIS QDs in the 3 to 8 nm size range (polydispersity of ~10%). The product CIS QDs can be readily overcoated by a thin (≤1 nm) wurtzite ZnS shell, resulting in CIS/ZnS core/shell QDs with PL tunable from the first to the second NIR-window (750 - 1100 nm) with high PLQYs (e.g. 75 % at 820 nm and 25 % at 1050 nm). These wurtzite CIS/ZnS core/shell QDs are

subsequently

transferred

to

water

upon

exchange

of

the

native

ligands

by

mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS core/shell QDs have PLQYs ranging from 39 % (at 810 nm) to 6 % (at 1030 nm) and good colloidal stability over at least 6 30 ACS Paragon Plus Environment

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months, and are thus promising candidates for in-vivo bioimaging applications. It should be noted that the hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shell overgrowth are also attractive as fluorophores in luminescent solar concentrators due to their large absorption crosssections throughout the UV-visible- NIR spectral range, high PLQYs (75% at 820 nm, which is an ideal wavelength in combination with Si solar cells), and large effective Stokes shifts, which would maximize harvesting of the solar spectrum while minimizing reabsorption losses.59-61

ASSOCIATED CONTENT Supporting Information. PL and absorption spectra of indocyanine green, TEM images and size histograms of Cu2-xS NCs in the 3 to 9.5 nm size range, optical spectra and TEM images of CIS QDs obtained under different synthesis conditions, XRD patterns and EDS spectra of Cu2-xS NCs and CIS QDs obtained by slow and fast CE, EDS spectra of CIS and CIS/ZnS core/shell QDs, representative PL decay curves of CIS QDs and CIS/ZnS QDs, FTIR spectra, TEM images, hydrodynamic size and Zeta potential of CIS/ZnS QDs before and after ligand exchange and phase transfer to water. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS Chenghui Xia would like to acknowledge China Scholarship Council (CSC) for financial support (NO. 201406330055). The authors thank Da Wang for TEM measurements, Gang Wang and Donglong Fu for XRD measurements, Ward van der Stam for PL lifetime measurements and suggestions concerning the cation exchange reactions, and Chun-Che Lin and Elleke van Harten 31 ACS Paragon Plus Environment


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for suggestions regarding the ligand exchange. The authors are also grateful to the Soft Condensed Matter group (Debye Institute for Nanomaterials Science, Utrecht University) for providing access to Fourier transform infrared spectroscopy, Zeta Potential and Dynamic Light Scattering measurements.

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