Visible Light Excitation-Induced Luminescence from Gold

Jun 16, 2019 - Visible Light Excitation-Induced Luminescence from Gold Nanoclusters Following Surface Ligand Complexation with Zn for Daylight Sensing...
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Article Cite This: Langmuir 2019, 35, 9037−9043

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Visible Light Excitation-Induced Luminescence from Gold Nanoclusters Following Surface Ligand Complexation with Zn2+ for Daylight Sensing and Cellular Imaging Chirantan Gayen,†,§ Srestha Basu,†,§ Upashi Goswami,‡ and Anumita Paul*,† †

Department of Chemistry and ‡Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India

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S Supporting Information *

ABSTRACT: Herein, we report a complexation reactionmediated extended aggregation of gold nanoclusters exhibiting luminescence under visible light excitation. The complexation reaction between the carboxylate groups of mercaptopropionic acid and zinc ions induced the aggregation of gold nanoclusters, which featured bright green luminescence upon excitation with visible light of wavelength 450 nm and beyond. This luminescence of aggregated Au NCs, easily discernible with bare eyes (under broad daylight excitation), was used as a probe for luminescence-based detection of molecules based on the pKa values of the latter. This aspect has been an unfilled dream of scientists pursuing research on the development of nanoscale sensors, as luminescence-based detection techniques offer a greater degree of accuracy and sensitivity compared to absorption-based methods, and was thus far an unexploited/untapped area by nanoscale materials. Moreover, facile imaging of mammalian cells was achieved using these aggregated clusters upon excitation with visible light. This study demonstrates the utility of luminescent nanoclusters, akin to organic dyes, as materials active under visible light excitation. Thus, the complexation reaction-based tailoring of the optical properties of nanoclusters served as an effective tool in pushing the absorption maxima of the nanoclusters from an ultraviolet to visible range, enabling the luminescence of nanoclusters under broad daylight excitation. Hence, the work embodied herein offers a unique route to widen the application potential of metal nanoclusters as sensors and bioimaging agents operating under visible light excitation.

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INTRODUCTION Luminescent metal nanoclusters (NCs) with atomic precision and uniform structural integrity provide a viable option for selective sensing of analytes and bioimaging at the best possible spatial resolution.1−10 Additionally, luminescent NCs have good photostability, large Stokes-shifted emission, and negligible cytotoxicity and hence are considered as suitable candidates for applications in sensing and bioimaging.11 However, the practical utilization of NCs as probes for molecules and cells has largely been untapped owing to the fact that the photoluminescence of NCs under visible light excitation is a barely known phenomenon. Materials exhibiting photoluminescence under visible and near-infrared light excitation are an intriguing class of materials owing to the inherent disadvantages associated with UV light excitation. Typical demerits of UV light as a probe lie in the fact that it damages cells, lacks selectivity of absorption by chromophores, and undergoes quick attenuation in tissues.12 On the other hand, visible light excitation offers a large window of absorption and is benign to biological systems. Also, barring the exception of vitamin D synthesis under UV light, a majority of biological processes are regulated by visible light.12 Moreover, sensors showing luminescence under visible light excitation are envisioned to be cost-effective and environ© 2019 American Chemical Society

mentally sustainable. On the other hand, successful development of nanoscale materials exhibiting photoluminescence under visible light has been an unfilled dream of scientists pursuing research on luminescence-based sensors as such detection techniques offer a greater degree of accuracy and sensitivity compared to absorption-based methods. In the current scenario, an emerging area of interest is the development of nanoscale materials exhibiting luminescence under visible light excitation.13−17 Interestingly, imaging of mammalian cells has been pursued under visible light excitation using gold NCs. 18 On the other hand, as demonstrated by our laboratory, a nascent entrant to the domain of NCs is their complexation reaction-mediated assemblies.19−23 Effectively, we have shown in earlier reports that the complexation reaction between the ligands stabilizing the clusters and additional metal ions lead to the formation of assemblies of clusters that are endowed with superior properties vis-à-vis the pristine clusters.19−23 However, an extended assembly of NCs with luminescence excitation under visible light was not observed in earlier cases. Nevertheless, the Received: April 4, 2019 Revised: May 19, 2019 Published: June 16, 2019 9037

DOI: 10.1021/acs.langmuir.9b00991 Langmuir 2019, 35, 9037−9043

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Figure 1. (A) Emission spectrum of (a) Au NCs and that following addition of ∼100 mg Zn acetate dihydrate at (b) 0, (c) 3, (d) 6, (e) 9, (f) 12, (g) 15, (h) 18, (i) 21, and (j) 24 min. The wavelength of excitation was set at 300 nm. (B) Excitation spectra of zinc-added Au NCs at (a) 0, (b) 3, (c) 6, (d) 9, (e) 12, (f) 15, (g) 18, (h) 21, and (i) 24 min. The wavelength of emission was set at ∼568 nm. (C) Emission spectrum of zinc-added Au NCs after ∼100 h of reaction between Au NCs and zinc acetate dihydrate. The wavelength of excitation was set at 450 nm. The inset shows the digital photograph of the dispersion of Zn Au NCs captured in broad daylight. (D) Corresponding excitation spectrum of Zn Au NCs. The wavelength of emission was set at 512 nm. Complexation Reaction between Gold NCs and Zinc Acetate Dihydrate. To the above formed dispersion of Au NCs, ∼100 mg of zinc acetate dihydrate was added, which resulted in the formation of a dispersion exhibiting bright luminescence at ∼575 nm. MTT-Based Cell Viability Assay. To check the viability of mammalian cells on exposure to Au NCs and Zn Au NCs, MTT (3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was carried out. The assay was conducted following the treatment of HeLa cells (1 × 104) with varying concentrations of Au NCs and Zn Au NCs for 24 h. After the treatment period, the MTT dye was added and the samples were incubated for 3 h which resulted in a purple color formazon formation, and its absorbance was measured at 550 nm based on the following formula:

development of such assemblies was deemed important for applications as sensors and bioimaging agents. Herein, we report a zinc-assisted aggregation of mercaptopropionic acid (MPA)-stabilized Au NCs exhibiting an unprecedented phenomenon of luminescence upon visible light excitation. The complexation reaction between zinc ions and the ligands stabilizing the Au NCs served as the key to achieve the extended aggregation of Au NCs, causing a reduction in the energy gap between the ligand-to-metal charge-transfer states and thereby pushing the absorption maxima to the visible range. Further, the luminescence of Zn Au NCs triggered by visible light was employed for the “bare eye” luminescence-based discrimination of molecules with varying pKa. Additionally, the aggregated Au NCs were also used for imaging mammalian cells under visible light excitation. It is worthwhile mentioning here that despite the several advantages of metal NCs over organic dyes, so far, the latter have only been used as stains for cells owing to their ability to be excited under visible light. However, the work embodied herein demonstrates a way to circumvent this obstacle toward the application potential of metal NCs.

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% of cell viability =

(A550 − A 655)sample (A550 − A 655)control

× 100

Confocal Laser Scanning Microscopy Studies. The viable dose after MTT assay was selected and confocal laser scanning microscopy (CLSM) was conducted with a Zeiss LSM 880 microscope. For imaging, first, the HeLa cells (10 × 103) were grown on a coverslip, and after attaining 70% confluency, they were treated with both Au NCs and Zn Au NCs for 3 h (incubated in a CO2 incubator, 37 °C), respectively. Following this treatment, the media were removed and the cells were fixed with 4% formaldehyde (10 min at 37 °C). The coverslips were then gently lifted with tweezers and placed upside down in a glass slide with its side sealed. Under CLSM, the samples were excited with a visible wavelength, that is, λex = 488 nm. Instruments. The luminescence (emission and excitation) spectra of all samples were acquired using a Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. The UV−vis spectra were recorded using a PerkinElmer Lambda-25 spectrophotometer. Digital photographs were captured using a smartphone (Lenovo vibe X3). Fourier transformed infrared (FTIR) spectra were acquired in a PerkinElmer, Spectrum One spectrophotometer. KBr was used for the preparation

EXPERIMENTAL SECTION

Materials. Tetrachloroauric acid, MPA, and potassium bromide were procured from Sigma-Aldrich. Zinc acetate dihydrate was purchased from Merck. All experiments and syntheses were done using Milli-Q water. All glasswares were thoroughly washed with aqua regia prior to the experiments and syntheses. Synthesis of Gold NCs. Gold NCs were synthesized by slightly modifying a procedure reported in our previous work. Briefly, to 10 mL water, 1 mL of HAuCl4 (10 mM) was added. To the resultant solution, 0.4 mL of MPA (0.11 M) was added. This led to the formation of a colorless dispersion exhibiting a weak luminescence at ∼575 nm. 9038

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Langmuir of pellets for FTIR analysis. CLSM images were acquired using a Zeiss LSM 880 confocal microscope.

dispersion, a broad excitation was observed ranging from 375 to 475 nm and beyond (Figure 1D). To understand the effect of zinc addition on the morphology of Au NCs, transmission electron microscopy (TEM) analysis of the dispersion containing zinc-added Au NCs was performed. As evident from the TEM analysis (Figure S4), upon interaction with zinc ions, Au NCs underwent aggregation into nanoparticles in the size range of 7−8 nm, consisting of NCs. The emission corresponding to 514 nm has possibly originated because of the formation of the extended assembly of Au NCs, mediated by Zn ions, which has lowered the energy difference between the excited and ground states and has also pushed the absorption maxima to the visible region. An evidence in support of this statement is further corroborated from lifetime measurements of Zn Au NCs. Surprisingly, the emission lifetime (comprising two components) corresponding to the peak maxima at 495 nm was found to be as high as 54 and 6 μs (Figure S5). This is attributed to thermally activated delayed fluorescence (TADF). In typical cases of TADF, the photoexcited electrons of the species undergo transition from a singlet excited state to a triplet state. However, owing to thermal activation, the system reverts to the singlet excited state, resulting in emission with the same energy as that of normal fluorescence, but with a longer emission lifetime.25 This phenomenon of aggregation of Au NCs upon addition of zinc ions, as evident from FTIR analysis (Figure S6), was further attributed to the chemical coordination between the carboxylate groups of MPA and zinc ions. To further affirm the aggregation of Au NCs upon interaction with zinc ions, timedependent UV−vis absorption spectra of the zinc salt-added Au NCs were acquired at various time intervals. As evident from Figure S7, the UV−vis absorbance spectra of zinc-added Au NCs featured the emergence of a new peak at ∼430 nm with an increase in time. As per literature reports, this observation of the gradual emergence of unprecedented peaks in the UV−visible absorption spectra of conjugated molecules has often been attributed to their aggregation.26,27 Thus, the gradual appearance of the new peak at 430 nm in the UV−vis absorption spectra of zinc-added Au NCs has been attributed to the aggregation of Au NCs following complexation with zinc ions. Thus, a zinc-coordinated assembly of Au NCs exhibiting bright photoluminescence upon visible light excitation was achieved. The nonluminescent/weakly luminescent Au NCs, upon interaction with zinc ions, were transformed into luminescent aggregates. This observation may be attributed to aggregation-induced emission. In the absence of zinc ions, the ligands stabilizing the weakly luminescent Au NCs, that is, MPA, were flexible under intramolecular rotations and vibrations. As a consequence, the nonradiative decay pathway might have been promoted, leading to vibrational and rotational relaxation. This led to lowering of the luminescence of Au NCs. However, upon complexation with zinc ions, the MPA molecules stabilizing the Au NCs underwent restricted intramolecular rotation and vibration. This is commonly referred to as a restricted intramolecular motion.28 Owing to the structural rigidity imposed on the Au NCs, following coordination with the zinc ions, the nonradiative decay pathway might have got blocked, and, instead, the radiative decay pathway got promoted, leading to enhancement in the luminescence of Au NCs. Given the bright photoluminescence of Zn Au NCs upon visible light excitation, chemical sensing of analytes using Zn Au NCs as a “visible light sensor” was deemed possible. It is

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RESULTS AND DISCUSSION Addition of MPA to an aqueous solution of tetrachloroauric acid resulted in the formation of a colorless dispersion with no discernible peak in the UV−visible absorption spectrum and exhibiting a weak luminescence at 575 nm upon excitation at 300 nm (Figure S1A,B). It is worth mentioning here that the formation of nonluminescent Au NCs upon reaction between HAuCl4 and MPA is not surprising, and our result matches well with that reported by others.24 Transmission electron microscopic analysis of this dispersion affirmed the formation of nanoscale particles with sizes around 2 nm (Figure S1C). To confirm the oxidation state of Au in Au NCs, X-ray photoelectron spectroscopy (XPS) analysis was performed. As evident from the XPS spectrum, two distinct peaks at 83.8 and 87.3 eV corresponding to Au(0) and Au(I) were observed. This suggested the presence of both Au(0) and Au(I) as the constituents of Au NCs (Figure S2). A recent research from our laboratory reports the synthesis of Au NCs using MPA and an additional ligand (e.g., histidine, tryptophan, tyrosine, cysteine, and mercaptobenzoic acid) as stabilizers.19−23 These NCs have further been used for the formation of a crystalline assembly of clusters for various applications. The presence of additional ligands was found to be essential for the complexation reaction-mediated crystalline assembly of NCs. However, in a work allied to ours, it has been shown that without the presence of these additional ligands, aggregation of NCs occurs, which lack crystallinity and hence are devoid of distinct structural features.24 In the next step, we were interested in pursuing the assembly of these NCs. As mentioned above, the earlier reported NCs from our group were used for versatile applications ranging from energy storage,19 chiral recognition, and separation20 to organelle-targeted theranostics.21 On the other hand, designing of an extended assembly of NCs with the optical properties unobtainable in individual NCs is an endeavor worth pursuing. In this regard, we wanted to achieve an emission from the assembly of NCs upon excitation in the visible region of light. It is worth mentioning here that the as-synthesized Au NCs were devoid of luminescence upon excitation in the visible range. Thus, the as-synthesized Au NCs were mixed with zinc acetate dihydrate. This led to a significant enhancement in the luminescence intensity of the weakly luminescent NCs upon excitation at 300 nm (Figure S3A,B). No significant enhancement of luminescence intensity was observed of Au NCs upon excitation with visible light, even immediately after the addition of zinc ions to a dispersion of Au NCs (Figure S3C). However, following incubation of the zinc-added Au NC dispersion for around an hour at room temperature, the color of the dispersion changed from colorless to greenish yellow (Figure S3D). Upon excitation of this dispersion at 300 nm, a gradual growth of a peak centered around 470−485 nm was observed with time. This was supplemented with a gradual decrease in the emission peak at 570 nm (Figure 1A). Similarly, the excitation spectra of the same dispersion (with the emission wavelength set at ∼568 nm) featured a decrease in intensity as well (Figure 1B). Upon excitation of this incubated yellow dispersion of the zinc-added Au NCs at 450 nm (visible light), an emission peak at 514 nm was observed (Figure 1C). On the other hand, upon fixing the emission at 514 nm and recording the excitation spectrum of this 9039

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Figure 2. (A1) Luminescence spectrum of (a) Zn Au NCs and that following the addition of (b) 980 μM of aspartic acid. (A2) Luminescence spectrum of (a) Zn Au NCs and that following the addition of (b) 980 μM of glutathione. (A3) Luminescence spectrum of (a) Zn Au NCs and that following the addition of (b) 980 μM of glutamic acid. (B1) Luminescence spectrum of (a) Zn Au NCs and that following the addition of (b) 980 μM of cysteine. (B2) Luminescence spectrum of (a) Zn Au NCs and that following the addition of (b) 980 μM of cystine. (B3) Luminescence spectrum of (a) Zn Au NCs and that following the addition of (b) 980 μM of tyrosine. (C) Schematic illustration of the plausible mechanism responsible for the change in the luminescence of Zn Au NCs upon interaction with various molecules. Excitation wavelength was set at 450 nm.

charge transfer, that is, the stabilizing ligands and the core metal clusters.29 Now, here, the MPA molecules being deprotonated will allow the facile ligand-to-metal charge transfer. On the other hand, if these MPA molecules are protonated, such a charge transfer may not be as facile as otherwise. The emission upon visible light excitation is believed to be a consequence of such a charge transfer owing to long-range interactions between the NCs mediated by Zn ions. Thus, with this knowledge, a system could be readily designed, wherein molecules with pKa less than the pKa of the stabilizer MPA and molecules having pKa greater than that of MPA could be differentiated based on a simple luminescence study. The feasibility of development of such a system was demonstrated using amino acids and peptides. As shown in Figure 2A1−A3, molecules bearing acidic side chains (amino acids: aspartic acid and glutamic acid, and tripeptide glutathione) upon interaction with Zn Au NCs led to a phenomenal shift in the emission peak of the latter when excited with visible light, that is, 450 nm. However, as seen in Figure 2B1,B3, no such shift was observed upon the interaction of molecules of amino acids such as cysteine, cystine, and tyrosine with MPA stabilizing the Zn Au NC aggregates. This

important to note here that based on previous results from our laboratory, we could demonstrate the specific reactivity of ligands on the surface of the assembled particles constituting the clusters. It has been observed and reported that the coordinating groups and the orientation of surface ligands play an important role in deciding the nature of interactions with the added molecules and thereby control the core properties (e.g., luminescence) of the assembly of clusters.20 Using this principle, the nanoscale assembly endowed with luminescence under visible light excitation, designed herein, was used as a probe for sensing molecules interacting specifically with the MPA ligands present on the surface of Zn Au NCs. The pKa of MPA as reported in the literature is 4.34. The pH of the typical dispersion of Zn Au NCs was recorded to be ranging from 5 to 5.9. Thus, at this pH, the surface MPA moieties were expected to exist in their deprotonated state, that is, as carboxylate end groups. Under this condition, any molecule with pKa less than that of MPA is supposed to act as a proton donor to MPA. On the other hand, molecules with pKa higher than that of MPA would not interact with the latter via acid−base reaction. It is worth mentioning here that previous reports claim that the luminescence of NCs primarily arises from the ligand-to-metal 9040

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Langmuir may be attributed to the fact that in case of aspartic acid and glutamic acid, the side chains, being acidic and having pKa lesser than that of MPA, might have protonated the MPA molecules on the surface of Zn Au NCs, thereby inhibiting the ligand-to-metal charge transfer and causing a significant change in the wavelength of emission. Similar is the case with glutathione which is again composed of glutamic acid as one of the component amino acid. On the other hand, the side chains of other amino acids like cysteine, cystine, and tyrosine are nonacidic and hence might have been noninstrumental in protonating the MPA molecules, as a consequence of which the emission of Zn Au NC aggregates remained nearly unaltered. As a proof of concept, we have shown additional examples (using other amino acids) of such selective interactions and their effect on the luminescence of Zn Au NCs under visible light excitation in Figure S8. The trend of change in the emission wavelength of Zn Au NCs upon interaction with the aforementioned analytes was also concordant with the change in the excitation spectra of Zn Au NCs. As shown in Figure S9, the excitation of Zn Au NCs in the visible range (acquired upon fixing emission at 512 nm) decreases drastically upon interaction with glutathione, glutamic acid, and aspartic acid. However, upon interaction with cysteine, cystine, and tyrosine, no discernible change in the visible light excitation of Zn Au NCs was observed. Also, the effects of glutamic acid, aspartic acid, glutathione, tyrosine, cysteine, and cystine on the UV−vis absorbance of Zn Au NCs were studied. In concordance with the trend obtained in the emission spectra of Zn Au NCs, the peak at ∼430 nm in the UV−vis absorbance spectrum of Zn Au NCs disappeared upon the addition of aspartic acid, glutamic acid, and glutathione (Figure S10A−C), whereas upon the addition of tyrosine, cysteine, and cystine, no significant change was observed in the peak at 430 nm (Figure S10D−F). This further suggested that the change in the emission wavelength of Zn Au NCs upon interaction with glutathione, aspartic acid, and glutamic acid might have occurred because of the ground-state interaction between Zn Au NCs and the respective analytes as opposed to the excited-state phenomenon. However, no specific pattern in the change of lifetime of Zn Au NCs was observed upon addition of the aforementioned analytes. Nevertheless, the values of the lifetimes of Zn Au NCs prior to and following interaction with the analytes are provided in the Supporting Information (Table S1). Another premise where significant advancements of luminescent materials, especially that of nanoscale particles, have been effective is that in bioimaging. As discussed in earlier sections, the domain of cellular imaging has significantly benefited from the emergence of luminescent NCs. However, NCs or their assembled systems, exhibiting luminescence under visible light excitation, poise to be a better choice than the existing systems. To this end, the system of zinc-assisted aggregation of Au NCs was used as a probe for imaging mammalian cells under visible light excitation. As evident from Figure 3A1−A3,B1−B3, upon treatment of HeLa cells with 66 μg/mL of Zn Au NCs and following incubation for around 3 h, bright luminescence could be observed upon excitation at 488 nm. On the other hand, upon treatment of HeLa cells with Au NCs (without zinc ions), no significant (very weak) luminescence was observed within HeLa cells upon excitation at 488 nm. It is important to note here that as per the MTTbased cell viability assay (Figure S11) at the concentrations of Zn Au NCs and Au NCs, at which cell imaging was performed,

Figure 3. (A1−A3) Representative CLSM images of HeLa cells treated with Zn Au NCs upon excitation at 488 nm. (B1−B3) Corresponding merged images of HeLa cells treated with Zn Au NCs shown in (A1−A3) acquired in CLSM mode and bright-field mode. (C1−C3) Representative CLSM images of HeLa cells treated with only Au NCs upon excitation at 488 nm. (D1−D3) Corresponding merged images of HeLa cells treated with Zn Au NCs shown in (C1− C3) acquired in CLSM mode and bright-field mode.

no significant killing of HeLa cells was observed. Further, the internalization of Zn Au NCs within HeLa cells was confirmed through depth projection analysis (Figure S12). The facile imaging of HeLa cells under visible light excitation using Zn Au NCs is an unprecedented application with regard to nanoscale particles. This is attributed to the long-range order charge transfer between gold clusters via zinc ions coordinated to MPA, stabilizing the clusters. To the best of our knowledge, so far no reports exist on nanoscale particle-based cellular imaging under visible light excitation.

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CONCLUSIONS In essence, we have demonstrated that zinc-mediated complexation reaction-based aggregates of Au NCs exhibit luminescence under visible light excitation. Further, the luminescence of aggregated Au NCs was used for mammalian cell imaging and as a probe for luminescence-based detection of molecules with varying pKa. Thus, as envisioned by Sakka and Sun,30 this is considered to be an important advancement in the field of nanoscale assembly of clusters as well as in the domain of development of sensors and bioimaging agents based on the visible light excitation-induced luminescence of these nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00991. UV−vis absorbance, emission, and XPS spectra of Au NCs; TEM image of Au NCs; emission, excitation, FTIR, and UV−vis absorbance spectra of Au NCs and those following the addition of zinc acetate dihydrate; delayed fluorescence emission spectrum of Zn Au NCs 9041

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and time-resolved delayed fluorescence decay curve of Zn Au NCs; luminescence spectrum of Zn Au NCs; MTT-based cell viability assay of Au NCs and Zn Au NCs; depth projection analysis performed on typical CLSM images of HeLa cells treated with Zn Au NCs; time-resolved photoluminescence analysis of Zn Au NCs; and STEM image of nanoaggregates of Zn Au NCs and elemental mapping of Au and Zn (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anumita Paul: 0000-0002-2999-9749 Author Contributions §

SB and CG contributed equally to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Electronics and Information Technology, Government of India, for financial help (no. 5(9)/2012-NANO (Vol. II)). Assistance from Mihir Manna and Archismita Hajra, Central Instruments Facility (CIF), IIT, Guwahati, is acknowledged. We are also thankful to the Indian Institute of Technology, Bombay, for the XPS facility.

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REFERENCES

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DOI: 10.1021/acs.langmuir.9b00991 Langmuir 2019, 35, 9037−9043

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Langmuir particles for Anticancer Gene Theranostics. ACS Appl. Mater. Interfaces 2014, 6, 712−724. (30) Sun, H.-T.; Sakka, Y. Luminescent metal nanoclusters: controlled synthesis and functional applications. Sci. Technol. Adv. Mater. 2014, 15, 014205.

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DOI: 10.1021/acs.langmuir.9b00991 Langmuir 2019, 35, 9037−9043