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Synthesis of Ultra - Stable Copper Nanoclusters and Their Potential Application as Reversible Thermometer Received 00th January 20xx, Accepted 00th January 20xx

a, b

Yu-e Shi, *a Wang







Shaojuan Luo, Xiaojing Ji, Fuwei Liu, Xian Chen, Yang Huang, Lei Dong, and Lei

DOI: 10.1039/x0xx00000x www.rsc.org/

A new strategy for the synthesis of luminescent copper nanoclusters (Cu NCs) was reported,by virtue of the reduction of Cu2+ using ascorbic acid with the protection of polyvinylpyrrolidone under 75 oC. Blue emitting Cu NCs, with photoluminescence (PL) quantum yield of 12% and high stability up to at least 1 month were obtained. Moreover, the PL of Cu NCs shows a reversible response to temperature and a liner relationship between PL intensity and temperature even after 10 cycles of repeated heating and cooling process was obtained, indicating great potential applications for thermal sensors.

Introduction Copper nanoclusters (Cu NCs) are tiny colloidal nanoparticles, containing few to some hundreds of Cu atoms, with core sizes 1-5 below 3 nm and capping ligands surrounded. Attributing to their extremely small size, some molecule-like properties, such as HOMO-LUMO transitions, molecular chirality, magnetism and photoluminescence (PL), deviate significantly from bulk metals and even nanoparticles.6, 7 Among those properties, PL is relatively attractive due to their advantages over common fluorophores, or other metal NCs (e.g. Au), including nontoxicity, low cost, high photostability and large Stoke shifts, which have been readily applied in the areas of chemical sensing, biological imaging, and light emitting devices.6, 8 However, some applications of this luminescent NCs have been curtailed by their relatively poor stability in aqueous solution, since their structure can be easily destroyed due to the nature of easy oxidation or aggregation, leading to the reduction of their PL. Therefore, Cu NCs with both strong PL and good stability are highly desired. Currently, the synthesis of Cu NCs is mainly based on a bottom-up method, where Cu 2+ ions are reduced to Cu atoms in solution, which will accumulate to clusters afterwards.9-13 To prevent further aggregation and improve stability of the

clusters, proper capping ligands, such as polymer, proteins, DNA, or small organic molecules, are delicately selected during 2, 6, 14 the synthesis process or post surface treatments. For example, Wang and co-workers reported the luminescent Cu NCs protected by polyvinylpyrrolidone (PVP), whose stability could be further improved by an effective post surface 15, 16 However, this treatment using electron-rich ligands. process is complicated and time-consuming, hampering its popularization and application in multiple fields. Here, we report a straightforward synthesis of Cu NCs by taking advantage of a high-temperature protocol, with ascorbic acid (AA) as reduction reagent and PVP as protecting ligands. The as-synthesized Cu NCs show strong blue emission with PL quantum yield of 12%, along with good stability in solution. Moreover, the resultant Cu NCs show rapid and reversible response to external temperature change even after multiple cycles, which demonstrates the potential application as a sensitive thermometer in various fields.

Experimental Materials All chemicals including copper nitrate (Cu(NO3)2), ascorbic acid, PVP, were of analytical grade and purchased from SigmaAldrich. Instruments Lambda 800 spectrophotometer (PerkinElmer, USA) and Shimadzu RF-5301 PC spectrofluorimeter (Shimadzu, Japan) were employed to measure absorption and PL spectra, respectively. Transmission electron microscopy (TEM) images were recorded on a TECNAI F20 (FEI, USA) operating at a voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG ESCALAB MKII spectrometer

J. Name., 2013, 00, 1-3 | 1

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(ThermoFisher Scientific, USA). PL lifetime measurements were performed on an Edinburgh FLS 980 Lifetime Spectrometer. Absolute PL QY was measured by a customized fluorescence spectrometer which is equipped with an integrating sphere (FLS920P, Edinburgh Instruments).

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Synthesis of Cu NCs Typically, 0.5 g of PVP was dissolved in 8 mL water, followed by 2+ the addition of 1 mL Cu solution (0.01 M) and 1 mL AA solution (0.1 M). After adjusting pH to 4.0, the mixture was o reacted for 3 h under 75 C. Finally, dialyzing process was conducted using a membrane for 24 hours. The product in the dialysis membrane was collected and stored at 4 °C for further use.

Results and discussion The luminescent Cu NCs are synthesized via a strategy as illustrated in Fig. 1a. In a typical synthesis, aqueous solution of 2+ Cu , AA and PVP were mixed, followed by adjusting the pH to 4.0 using NaOH. Then, the mixture was sealed and heated up o to 75 C for 3 h. In this system, PVP provides binding sites for 2+ Cu based on the interaction of single nitrogen atoms and the 2+ 17 empty orbitals of Cu (Fig. 1a). AA as a mild reduction 2+ reagent is used to reduce Cu into Cu atoms, and 18 subsequently clustering on the framework of PVP. The mixture changed from colourless to light yellow and showed bright blue emission under UV light, indicating the formation of Cu NCs. To directly observe Cu NCs, TEM measurements were performed, as shown in Fig. 1b. Uniformly dispersed clusters with size around 3 nm were observed, without any large aggregation or Cu nanoparticles. The size distribution of Cu NCs is also presented in the inset of Figure 1b, showing an average size of 3.3±0.8 nm. This is consistent with the data 16 from Wang and co-workers. The size of as-synthesized Cu NCs is bigger than commonly reported NCs in literature with the diameters around 1.8 nm. However, as reported by Zhou 9 and co-workers, an increased size of Cu NCs, from 1.8 to 3.1 nm, would appear after the addition of PVP, since PVP would wrap onto the surface of NCs. XPS analysis was carried out to study the oxidation state of Cu. As shown in Fig. 1c, the 2+ absence of peak at 942 eV indicates the fully reduction of Cu , whereas strong peaks at 932 and 953 eV can be ascribed to the binding energy of 2p3/2 and 2p1/2 electrons of Cu atoms, 6 respectively. Fig. 1d presents the UV-visible absorption spectrum of Cu NCs. An obvious absorption peak at 380 nm and a shoulder at 440 nm can be observed, which are related 19-21 to the inter-band electronic transitions of clusters. No surface plasmon resonance absorption peaks (500 – 600 nm) are observed, suggesting the absence of large copper 22 particles. In addition, bright blue emission (peaked at 425 nm) is recorded, with well pronounced PL excitation peak at 380 nm, which is in good agreement with previous reports 18, 23 about the optical properties of Cu NCs. To get more information on the origin of PL in Cu NCs, PL decay curves were measured as well. As shown in Fig. 1e, the curve can be

fitted by three-exponential function, with three components of 1.0 ns (38.1%), 3.2 ns (56.6%), and 8.0 ns (5.3%). The average PL lifetime is calculated to be 2.8 ns, attributed to the emission 5, 24 from singlet excited state. The conditions for synthesis, such as pH, concentration of AA, reaction time and temperature, play an important role on the PL of Cu NCs. The pH of reaction system could act on the 2+ reduction ability of AA towards Cu . As shown in Fig. 2a, a maximum PL intensity is obtained under the pH of 4.0 and deviating values lead to a decline of PL, even almost nonemission under pH of 7.0. This can be explained by the higher reduction ability of AA under alkali conditions that leads to the formation of non-luminescent bigger-sized clusters. Similar to the effect of pH, concentration of AA can also affect the PL intensity by tuning its reduction ability, and an optimized intensity was achieved at the concentration of 0.4 M. What’s o more, a reaction time of 3 h under 75 C had already given us a maximum PL intensity, while no obvious variation was observed with extra 2 h. Thus, our synthesis is much less time 15 consuming, compared with previous reports (3 h vs 1 week), which can be attributed to the proposed high temperature synthesis strategy that accelerates the reaction dynamics. Actually, reaction temperature would not only boost the reaction but also impact on the PL intensity of the product (Fig. 2d), because insufficient reduction or slight aggregation of NCs might occur when the temperature is too low or too high. Under the above optimized conditions, blue emitting Cu NCs exhibit a PL quantum yield (QY) of 12%, which is in the top 1, 6 range of Cu NCs. The stability of Cu NCs in solution, especially in the PL 25-27 property, is a key factor for their practical applications. Only several recent papers reported Cu NCs with both strong 13, 15, 16 PL and good stability. One good example was reported by Wang and co-workers, achieving high stability as long as 1 15 month. However, long reaction time (1 week) and tedious post-treatment were needed. Whereas, in our straightforward method, the synthesis of high QY Cu NCs only takes 3 hours while avoiding the post-treatment. Besides, our as-synthesized Cu NCs are extremely stable in solution that would improve their practicability. Fig. 3a shows the UV-visible absorption and PL spectra of Cu NCs before and after long-term storage. No obvious change was observed (

Synthesis of ultra - stable copper nanoclusters and their potential application as a reversible thermometer.

A new strategy for the synthesis of luminescent copper nanoclusters (Cu NCs), by virtue of the reduction of Cu2+ using ascorbic acid and the protectio...
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