Enhanced Luminescence of a Quantum Dot Complex Following

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Enhanced Luminescence of Quantum Dot Complex Following Interaction with Protein for Applications in Cellular Imaging, Sensing and White Light Generation Satyapriya Bhandari, Sabyasachi Pramanik, Naba Kumar Biswas, Shilaj Roy, and Uday Narayan Pan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00233 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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ACS Applied Nano Materials

Enhanced Luminescence of Quantum Dot Complex Following Interaction with Protein for Applications in Cellular Imaging, Sensing and White Light Generation Satyapriya Bhandari,1,2,* Sabyasachi Pramanik, 3 Naba Kumar Biswas, 3 Shilaj Roy, 3 Uday Narayan Pan 3

1Centre

2Centre

for Nanotechnology, Indian Institute of Guwahati, Guwahati-781039, India.

for Nano and Material Sciences, JAIN (Deemed to be University), Jain Global Campus, Bangalore 562112, India.

3Department

of Chemistry, Indian Institute of Guwahati, Guwahati-781039, India. *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Herein, we report the sustainable, cost-effective and greener surface modification strategy of quantum dot complex (QDC) – comprised of Zn-chalcogenide quantum dot (Qdot; for example undoped ZnS and Mn2+-doped ZnS) and surface Zn-quinolate (ZnQ2) complex – following the interaction with bovine serum albumin (BSA) protein. The interaction of BSA to QDC led to enhancement in their luminescence properties (such as quantum yield and emission life time), solubility and stability (in water), compared to their bare form. The enhanced luminescence properties of BSA coupled QDC – also termed herein as BSA-QDC nanocomposite – is due to the BSA induced structural rigidity of the ZnQ2 complex (being present on the surface of Znchalcogenide Qdot). The highly green luminescent nontoxic BSA-QDC nanocomposite – consisted of undoped ZnS and ZnQ2 complex – showed better imaging capability of HeLa cells in comparison to only QDC and sensing ability of an enzyme (trypsin) with a detection limit of 0.06 M. Interestingly, the interaction of BSA to a dual emitting QDC (consisted of orange emitting Mn2+-doped ZnS Qdot and green emitting surface ZnQ2 complex) resulted the generation of white light (which is hard to achieve from only dual emitting QDC at ex-330 nm), with chromaticity coordinates of (0.33, 0.35) and (0.29, 0.35), color rendering index (CRI) of 86 and 80, and correlated color temperature (CCT) of 5646 and 7377 K, in liquid and solid phases respectively. This may open up a new paradigm towards sustainable and user-friendly surface modification strategies for fabricating advanced nanoscale materials, with anticipated uses in imaging, sensing and light emitting applications. KEYWORDS.


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Quantum dots, inorganic complex, protein, bioimaging, sensing, white light emission.


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1. Introduction

The surface modification of colloidal quantum dots (Qdots), using different chemical and biological moieties, is of paramount interest for fabricating newer and ecofriendly multifunctional nanocomposites, with superior optical features (such as high quantum yield (QY) and longer emissive life time), better solubility and stability in aqueous medium. 1-35 This led to the versatile and sustainable usages of surface modified Qdots in the field ranging from bioimaging to biosensing to white light emitting diodes (LEDs).1-35 Till date, several surface modification strategies (such as ion exchange, ligand exchange etc.) have been reported for improving the stability, optical performance and application potential of the Qdots.1 -35 Among all the reported surface modification strategies of Qdots, the formation of inorganic complex on the surface of Qdots have become a greener, simpler and cost-effective strategy, which is uniquely different from ordinary ligand exchange strategy, to alter their luminescence characteristics as well as application potential of Qdots.

28-35

The inorganic complex coupled Qdot, termed as quantum dot complex

(QDC), showed superior optical features compared to their parent components (i.e. Qdot and inorganic complex) and have demonstrated their usages in fabricating targeted bioimaging agent, white light emitting (WLE) materials and sensing of human disease’s responsive molecules. 28-35 However, the surface modification of the QDC (comprised of inorganic complex and Qdot) in order to make further advanced and superior luminescent multifunctional nanoprobe, with enhanced luminescence and solubility and stability and an aim to explore their multiple applications in the fields ranging from bioimaging to biosensing to white light generation, is not yet explored. Alternatively, acquiring enhanced luminescence properties of the ZnQ2 complex (being present on the outer surface of the Zn-chalcogenide Qdots) – which is solely responsible for the luminescence of QDC – followed by activating their surface states and accomplishing of


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structural rigidity via the interaction with different chemical or biological means, is the prime aim of the current work. To best of our knowledge, there is no report on the surface modification of the QDC (comprised of Zn-quinolate complex and Zn-chalcogenide Qdot), using chemical and/or biological means, with an aim to explore their enhanced luminescence, solubility and stability, and thus towards multiple applications including bioimaging, biosensing and light emitting purposes. This will not only open up an innovative dimension to find out the different greener and sustainable chemical strategies to modifying the surface of the newer nanocomposites (alike QDC) but also convey a new chemical message towards unravelling surface chemistry of the QDC. Hence, it would be noteworthy to modify the surface of QDC, using chemical and/or biological means, for synthesizing newer and advanced multifunctional nanocomposites with enhanced optical properties, solubility and stability and consequently showing their anticipated applications in bioimaging, biosensing and white LEDs. Importantly, protein, with various chemical functionalities, have been demonstrated their use as a source of fabricating nonmetallic Qdots, template to synthesize biocompatible metal based Qdots and nanoclusters, and also served as a surface modifying agents for improving the solubility and optical features of different nanoscale emitters. 36-41 Henceforth, protein based nanomaterials for functional devices have gained remarkable attention in this regard.1 Among the existing members of the protein family, bovine serum albumin (BSA) protein have been extensively used in the field of nanobiotechnology.36-41 Thus, it would be significant to use BSA as a surface modifying agent in order to improve the optical quality, in addition to solubility and stability, of QDC to make them advanced nanocomposite for cellular imaging, enzyme sensing and fabricating single component WLE material.


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The real applicability of any luminescent nanoprobe for cellular imaging and biosensing purposes entirely depending upon their luminescence properties (such as quantum yield (QY) and emission life time), solubility, stability and environment sustainability. Notably, the higher QY and longer emission life time of any nanoprobe (especially green luminescent one) help in minimizing and/or avoiding the optical interferences of the cells during imaging process as well as to sense analyte with a required detection limit – for specific practical purposes.1-20 Henceforth, there is always a need for fabricating green luminescent nanoprobe, with enhanced optical features, for bioimaging and biosensing purposes. Importantly, probing the activities of the enzymes is of supreme concern for careful diagnosis of the human diseases - such as cancer, cardiovascular, neurodegenerative and inflammation – which are usually caused due the unusual content of respective enzymes.16-20 As a member of the enzyme family, trypsin - an important digestive enzyme – plays a vital role to control pancreatic exocrine function and can specifically hydrolyze the C-terminal of arginine and lysine of proteins.16-20 Abnormal content of trypsin may cause human diseases such as pancreatitis and meconium ileus, cystic, brosis and apoptosis etc.16-20 While various nanoprobe and techniques (such as mass spectrometry or electrochemical or gel electrophoresis methods) have been used for sensing of trypsin; however, their application is limited either due to the concern of toxicity or time consumption, complicated procedure and high cost.16-20 Hence, it would be highly desirable to fabricate a new, cost-effective, greener and advanced optical nanoprobe for the detection of trypsin (with a detection limit below than 84.4 g/mL (3.55 M) - which is the average trypsin concentration in pancreas transplant patients as per earlier reports)17 and thus to their future practical uses. On the other hand, the fabrication of highly efficient and stable Qdot-based optoelectronic devices (such as light emitting devices (LEDs), displays and photovoltaic devices etc.) is the prime


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concern of the current scientific communities for partial mitigation of the global energy problems.21-31 In this regard, the fabrication of an inexpensive highly efficient single component environmentally sustainable white light emitting (WLE) material, compared to multicomponent system based WLE materials (which are usually suffered from the problems related to undesirable change in chromaticity, self-absorption and nonradiative energy transfer) have extended vital concern.21-31 In other words, there is a need to develop single component WLE material, with properties close to day bright light in terms of achieving perfect color chromaticity coordinates (0.33, 0.33), high color rendering index (CRI>80) and correlated color temperature (CCT) near to 6000-6500 K, for the alleviation of energy problems.21-31 In this respect, various nanomaterials have been demonstrated their use to fabricate single component WLE materials. Among them, the Qdot-based single component WLE materials have gained supreme interest, due to their extraordinary optical features and stability, and thus shaped the recent focus of the scientific community towards constructing Qdot-based optoelectronic devices with an aim to mitigate the energy consumption problem.21-31 Incidentally, various chemical strategies have been implemented in order to make a single component Qdot- based nanocomposite – which could be able to emit white light in liquid and solid phases, respectively.21-31 For example, complexing the surface of Qdots presented in protein matrix - consisted of gold clusters - have already paved their way towards generation of white light at a single wavelength of excitation.31 However, the nonradiative energy transfer between two nanocrystals present in the protein matrix and complicated fabrication procedure (followed by synthesizing two nanocrystals in a same protein matrix including complexation reaction) limited their user-friendliness and consequently application potential.31 Thus, it is always a novel thrust to find out a new, sustainable, user-friendly and advanced chemical strategy to make an innovative cost-effective bright Qdot-based WLE


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sources – which emit white light with chromaticity color coordinates, CRI and CCT close to the perfect bright day light emitters.21-31 Herein we report the surface modification of a presynthesized QDC following interaction with BSA and consequently demonstrated the multiple application of BSA coupled QDC in imaging cancerous HeLa cells, sensing of a digestive enzyme (trypsin) and generation of white light emission (with properties close to day bright light). The imaging and sensing application of were demonstrated by using green emitting QDC – composed of ZnS Qdot and ZnQ2 complex – following interaction with BSA while BSA interacted dual emitting QDC (comprised of Mn2+ doped ZnS Qdot and ZnQ2 complex) demonstrated their application in white light generation. The BSA protein coupled QDC is named herein as BSA-QDC nanocomposite. Importantly, the 2-fold enhancement in optical properties (especially quantum yield and emission life time) of green luminescent QDC (i.e. ZnQ2 attached ZnS Qdot), following interaction with BSA protein, was noted. The structural rigidity and activation of the surface states of the ZnQ2 complex (present in QDC), following Langmuir type binding isotherm with BSA, may be attributed for the superior optical characteristics of the BSA-QDC nanocomposite compared to individual QDC. Remarkably, the use of biocompatible photostable BSA-QDC nanoprobe (composed of ZnQ2 attached ZnS Qdots and BSA protein) as a bioimaging agent to image human cervical cancer HeLa cells and as an effective and selective sensing platform for trypsin, with a detection limit of 0.06 M, have been demonstrated. Importantly, using the same BSA coupling strategy to a dual emitting QDC (composed of orange emitting Mn2+ doped ZnS Qdot and green emitting surface ZnQ2 complex) resulted the fabrication of a single component nanocomposite, which displayed bright cool white light (at ex-330 nm) with chromaticity coordinates (0.33, 0.35) and (0.29, 0.35), CRI of 86 and 80 and CCT of 5646 and 7377 K in their liquid and solid phases, respectively while it is hard to


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achieve perfect white light emission from only dual emitting QDC (at ex-330 nm). This is the first report showing the surface modification strategy of QDC (composed of surface Zn-quinolate complex and Zn-chalcogenide Qdot) following interaction with protein and thereby utilization of the strategy for multiple applications ranging from cellular imaging to sensing to white light generation. 2. Experimental Section The details of the synthesis and charcaterization of ZnS Qdots and QDC (i.e. ZnQ2 complex attached ZnS Qdots) – followed by an earlier reported method33 – are clearly demonstrated in the Supporting Information. The ZnS Qdots and QDC are characetrized by powder x-ray diffraction (XRD), transmission electron microscopic (TEM), high resolution TEM analyses, UV-vis and fluoresence spectroscopic measurements. In brief, the presented results clearly demonstrated the succesful formation of pale green luminescent QDC (with abs -320 and 365 nm and em-510 nm) from a blue luminescennt undoped ZnS Qdots (with abs -320 and em- 440 nm), following complexation reaction with 8-hydroxyquinoline (HQ), without altering the morphology, intregrity of the Qdots (Figure S1 and Figure S2, Supporting Information). The experimental details of the (i) surface modification strategy of QDC (comprised of ZnS Qdots and surface ZnQ2 complex) followed by interaction with BSA and (ii) their use in bioimaging and biosensing (sensing of trypsin) are clearly described in the Supporting Information. Additionally, the deatils of the fabrication of WLE material - following the same protein coupling startegy for a presynthesized dual emitting QDC (comprised of Mn2+ doped ZnS Qdot and ZnQ2 complex) 30 – is also described in the Supporting Information.


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3. Results and discussion Scheme 1. Schematic illustration of the applications of (A) a highly luminescent green BSA-QDC nanoprobe (fabricated based on the interaction between BSA and a presynthesized QDC – composed of ZnS Qdot and ZnQ2 complex) as a bioimaging agent and an optical sensor for trypsin. (B) Schematic illustration of the formation of a single component white light emitting nanocomposite, using the same BSA protein coupled interaction with a dual emitting QDC – consisted of an orange emitting Mn2+ doped ZnS Qdot and green emitting surface ZnQ2 complex.

Scheme 1A elucidates the interaction between a presynthesized QDC (composed of ZnS Qdot and ZnQ2 complex) and BSA protein leading to formation of a highly luminescent nontoxic nanocomposite – termed herein as BSA-QDC nanocomposite – with enhanced luminescence properties (such as high QY and longer life time) compared to


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as such QDC. Importantly, BSA coupled QDC (composed of ZnS Qdot and ZnQ2 complex) endows their uses as a better imaging agent of human cervical cancer HeLa cells in comparison to only QDC at lower laser power as well as an environment-friendly optical sensor of trypsin (followed by enzymatic digestion and subsequent removal of BSA from the surface of QDC). Interestingly, the strategy of coupling of BSA to a presynthesized dual emitting QDC (comprised of an orange emitting Mn2+ doped ZnS Qdot and green emitting surface ZnQ2 complex) extended their use in generation of white light emission – which is as depicted in scheme 1B.

3.1. BSA-QDC (ZnQ2 complexed ZnS Qdots) Formation and Characterization.

Figure 1. (A) Digital photographs (under 365 nm light captured with a spectrofluorimeter) and (B) emission spectra (ex-365 nm) of the aqueous dispersion of the (i) QDC (pH-6.8; composed of


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ZnS Qdot and ZnQ2 complex) and (ii) BSA-QDC (pH-6.9) nanocomposite. (C) Emission spectra (ex-365 nm) of the aqueous dispersion of QDC (composed of ZnS Qdot and ZnQ2 complex) followed by addition of different amount (i) 0.0, (ii) 1.5, (iii) 2.9, (iv) 4.4, (v) 5.8, (vi) 7.1 and (vii) 8.5 M of BSA protein and (D) corresponding Langmuir type binding isotherm obtained from the plot of [concentration of BSA/I] (where I = change in the emission intensity at 500 nm of QDC following BSA addition and concentration of BSA is in the range of 1.5-8.5 M). (E) Time resolved photoluminescence decay profiles of (i) QDC and (ii) BSA-QDC nanocomposite. (F) UV-vis spectra of the aqueous dispersion of the (i) QDC (pH-6.8; composed of ZnS Qdot and ZnQ2 complex) and (ii) BSA-QDC (pH-6.9) nanocomposite. (G) Circular dichroism (CD) spectra of (i) native BSA (pH-6.9) and (ii) BSA-QDC (pH-6.9) nanocomposite. (H) MTT-based cell viability assay of human embryonic kidney HEK 293 cells with varying concentrations of BSA−QDC nanocomposite (after 24 h treatment). Digital photographs (captured under the 365 nm light from a spectrofluorimeter) showed that the treatment of BSA to the aqueous dispersion of the QDC (comprised of ZnS Qdots and surface ZnQ2 complex) - following centrifugation and redispersion - resulted the change in the luminescence color from pale green to bright green (Figure 1A). Interestingly, the 2-fold enhancement in emission intensity, with a shift in emission maxima from 510 to 500 nm, of QDC (having absorbance of 0.04 at 365 nm), followed by interaction with BSA, was noticed (Figure 1B). Similar enhancement in excitation intensity at 365 nm (without any shift in excitation maximum) was observed when BSA treated with QDC (Figure S3, Supporting Information). There is no significant change in pH was observed when BSA treated to QDC and thus ruling out the possibility of pH effect on the observed changes in emission characteristics of QDC following BSA addition. While upon excitation at 280 and 365 nm, only BSA showed emission peaks at 365


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and 450 nm, respectively (Figure S4, Supporting Information). It is to be mentioned here that the optimum concentration of BSA was calculated followed by observing the maximum emission intensity at 500 nm (Figure 1C). The optimum concentration of BSA was found to be of 8.5 M. The BSA protein coupled QDC is termed herein as BSA-QDC nanocomposite. Further, the attachment of the BSA to the surface of QDC and the purification of the BSA-QDC nanocomposite was confirmed followed by observing the emission intensity of the pellet obtained after centrifugation and redispersion of the 8.5 M BSA treated QDC mixture (Figure S5, Supporting Information). On the other hand, no significant change in the emission characteristics of only ZnS Qdots, followed by treatment of BSA, was observed (Figure S6A, Supporting Information) while addition of BSA to only ZnQ2 complex resulted alike enhancement in emission intensity as is the case when QDC treated with BSA (Figure S6B, Supporting Information). Earlier reports showed that the enhancement in emission intensity of only ZnQ2 complex, following interaction with proteins, could be possible (also tested here; Figure S6B, Supporting Information) and the protein concentration dependent enhancement of emission intensity of ZnQ2 was efficiently described by Langmuir type binding isotherm and the activation of their surface states followed by gaining structural rigidity.41 Notably, a linearity (with R2=0.99) was observed when the ratio of concentration of BSA and I (where I = change in the emission intensity of QDC (at 500 nm) following BSA addition) was plotted against concentration of BSA (in the range of 1.5-8.5 M; Figure 1D). The details of the Langmuir type binding isotherm is described in the experimental section. This clearly indicated that the binding of BSA to the surface of QDC could be effectively described by Langmuir type binding isotherm and consequently, the attainment of structural rigidity and activation of the surface states of the surface ZnQ2 complex (in BSA-QDC) – may be the responsible factor for the enhanced emission characteristics of BSA-QDC compared to


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individual QDC. As per earlier reports, the electronic transition from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of the ZnQ2 complex (present in QDC) is the origin of the luminescence of QDC and thus Qdot supported rigidity of the ZnQ2 complex helped them to show superior optical features compared to ordinary ZnQ2 complex.30-34 Thus, it can be concluded here that the structural rigidity induced activation of the surface states of the surface ZnQ2 complex (present in the QDC) - in presence of BSA - is playing role for the enhancement in the luminescence properties. Importantly, 2-fold enhancement in the photoluminescence quantum yield (PLQY; with respect to quinine sulphide) was noticed when BSA reacted with QDC. Notably, BSA-QDC exhibited PLQY of 7.5% while 3.4% PLQY was observed for only QDC (Table S1, Supporting Information). As is clear from the time resolved photoluminescence decay profiles (using 375 nm laser) analysis, the average life time of QDC, followed by interaction with BSA, increased from 10.4 ns to 16.1 ns (Figure 1E, Table S2, Supporting Information). Interestingly, the aqueous dispersion of BSA-QDC was found to be photostable with respect to their emission maxima at 500 nm and under continuous irradiation of 365 nm light (Figure S7A, Supporting Information). Additionally, the stability of the BSA−QDC in terms of their time dependent luminescence (at λem-500 nm) in water medium for 48 hours was noticed and thus demonstrated their biocompatibleness and future practical use (Figure S7B, Supporting Information). Therefore, the enhanced quantum yield, emission life time of the photostable BSA-QDC (comprised of ZnQ2 complexed ZnS Qdots) make them an attractive candidate for bioimaging (especially to avoid cellular auto-fluorescence) and sensing of important enzymes (which could be able to break the bonding between BSA and QDC). Further, the appearance of a new absorption peak at 280 nm (which is due to the BSA protein)31 – in addition to the original absorption peaks of QDC at 320 nm (due to the excitonic band gap of ZnS Qdot)33


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and 365 nm (due to the surface ZnQ2 complex)33 – of BSA added QDC (following centrifugation an purification) clearly indicated the successful formation of BSA-QDC nanocomposite (Figure 1F). Importantly, circular dichroism (CD) spectroscopic analysis was used to confirm the effective formation of BSA-QDC nanocomposite followed by observing the conformational changes in the BSA protein (Figure 1G). Notably, a positive peak at 195 nm and the two characteristics negative peaks at 210 and 222 nm – due to the secondary structure of the protein31, 42– were observed for native BSA (in water; pH-6.8). Upon interaction of BSA with QDC (followed by centrifugation), the characteristics peak at 195 nm shifted to 192 nm and little expansion in the peaks at 210 and 222 nm (with 2 nm red shifts) were noticed, compared to their native form (Figure 1G). This may be due to the changes in secondary structures of BSA when interacted with QDC. Additionally, the occurrence of the CD signals in the BSA-QDC nanocomposite (following centrifugation) clearly supported the presence of the BSA on the surface of the QDC. There is no significant changes in the structural integrity and morphology of the QDC (particularly ZnS Qdot), following interaction with BSA, were observed (Figure S8, Supporting Information). Precisely, the uniform compositions and cubic morphology of the ZnS Qdots (which was previously preserved during the formation of QDC and clearly demonstrated in Figure S1 and Figure S2, Supporting Information) – remained unaltered even after the formation of the final product (BSA-QDC) – which were confirmed followed by observing their similar diffraction patterns, with peaks at 28.6o, 48.3o and 56.2o (due to (111), (220), (311) Bragg’s lattice planes of cubic crystal of ZnS Qdots)30-34 and lattice fringe of 0.3 nm (which is due to the (111) plane of the cubic ZnS Qdots) 30-34 in the powder x-ray diffraction analysis and high resolution transmission electron microscopic (TEM) measurements (Figure S8, Supporting Information). This clearly indicated the surface modification of QDC, followed by interaction with BSA, without affecting the morphology of the


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QDC and thus demonstrated the successful fabrication of a highly luminescent BSA-QDC nanocomposite. Interestingly, the nontoxic nature of the BSA-QDC nanocomposite – which is important aspect to describe their environment-friendliness for real life applications (such as cellular imaging, sensing and white light generation) – was tested using MTT-based cell viability assay with the help of human embryonic kidney HEK 293 cells. Interestingly, more than 95.0% of the human embryonic kidney HEK 293 cells were found to be viable when different concentration of BSA-QDC (in the range of 10-120 g/mL) incubated HEK 293 cells for 24 hours (Figure 1H). This clearly supported the nontoxic nature of the BSA-QDC nanocomposite and thus demonstrated their suitability and applicability for aforementioned multiple applications. 3.2. Cellular Imaging of BSA-QDC (ZnQ2 complexed ZnS Qdots)


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Figure 2. Confocal laser scanning microscopic images (CLSM: scale bar - 20 μm) using different laser power of (A) 30 and (B) 4 mW of HeLa cells followed by 2 hours incubation with (i) BSA−QDC and (ii) QDC nanocomposite. Importantly, the highly green luminescent BSA-QDC nanocomposite, following two hours incubation with cervical cancer HeLa cells, exhibited bright green fluorescence (with intensity of 19.746 a.u.) under confocal microscope, using 355 nm laser and using laser power of 30 mW (Figure 2A). On the other hand, in absence of BSA-QDC, only HeLa cells did not exhibited any such green fluorescence (Figure S9, Supporting Information). Notably, Z-stake images in both depth and orthogonal projection confirmed the internalization of BSA-QDC in HeLa cells (Figure S10, Supporting Information). It is to be mentioned here that the quality of a luminescent bioimaging nanoprobe, especially green luminescent one, completely depends upon their quantum yield and emission life time. Notably, high quantum yield and longer emission life time are the necessary factors to minimize and/or avoid the optical interferences of the cells during imaging process using any green luminescent nanoprobe. Thus, the 2-fold enhanced quantum yield and longer emission life time of green luminescent BSA-QDC nanocomposite, compared to as such QDC, make them suitable and advantageous for cellular imaging. Further, the presence of the BSA protein at the outer surface of the BSA-QDC nanocomposite, in comparison to only QDC, make cellular internalization much more feasible followed by enhancing their stability and solubility in aqueous medium. This clearly demonstrated the use of green emitting BSA-QDC, with enhanced optical features (especially high quantum yield and loner emission life time – which are essential factors to avoid auto fluorescence of the cells) and ease of cellular internalization, as an excellent


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bioimaging green luminescent nanoprobe. Notably, only QDC, following two hours incubation with cervical cancer HeLa cells, showed green fluorescence (with intensity of 9.595 a.u.) under confocal microscope using 355 nm laser and using laser power of 30 mW (Figure 2B). Interestingly, at lower laser power of 4mW, BSA-QDC exhibited strong green fluorescence while weak green fluorescence was observed for only QDC (Figure 2A-2B). This clearly indicated the higher luminescence quantum yield of the BSA-QDC nanocomposite, in comparison to only QDC, offered the better imaging capability for labeling the HeLa cells even at lower laser power. 3.3. Sensing of Trypsin by BSA-QDC (ZnQ2 complexed ZnS Qdots)

Figure 3. (A) Emission spectra (λex - 365 nm) of (i) 0.26, (ii) 0.52, (iii) 0.78, (iv) 1.04, (v) 1.30, (vi) 1.55, (vii) 1.81, (viii) 2.06, (ix) 2.31 (x) 2.56 and (xi) 2.81 M of trypsin added to BSA-QDC nanocomposite (having absorbance of 0.04 at 365 nm) and (B) corresponding plot of quenching of emission intensity (at 500 nm) of BSA-QDC versus concentration of trypsin. (C) Quenching


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(%) of emission (at 500 nm) of BSA-QDC followed by addition of (i) trypsin (2.86 M), (ii) water (blank), (iii) L-Lysine (50.0 M), (iv) L-Cysteine (50.0 M), (v) glucose (50.0 M), (vi) Glycine (50.0 M), (vii) glutathione (50.0 M), (viii) urea (50.0 M), (ix) lysozyme (7.2 M), (x) GOx (22.0 M), (xi) Ca2+ (50.0 M), (xi) Zn2+ (50.0 M), (xi) K+ (50.0 M), (xi) Na+ (50.0 M). The sequential addition of trypsin to BSA-QDC nanocomposite resulted gradual quenching of their green emission (at 500 nm; Figure 3A). This was demonstrated followed by adjusting the pH of the solution of BSA-QDC at 8.0 (using phosphate buffer) and maintaining the temperature at 32oC – which are essential factors for trypsin assisted cleavage, especially 59 lysines and 24 arginines residues, of BSA (with practical efficiency).16-20 The plot of quenched luminescence intensity (I at 500 nm) of BSA-QDC against the trypsin concentration was found to be linear in the range of 0.26-2.81 M (Figure 3B). Thus, the BSA-QDC nanocomposite could be able to detect trypsin with a detection limit of 0.06 M – which is quite suitable for the detection of trypsin, as per earlier reported observations.16-17 Notably, as per earlier reports, the average trypsin concentration is 84.4 g/mL (3.55 M) in pancreas transplant patients.17 Hence, the proposed BSA-QDC nanoprobe has the required ability to sense trypsin, which is quite below compared to the limit of any pancreas transplant patient and thus BSA-QDC may be quite suitable for specified practical purposes. Additionally, the non-cytotoxicity of BSA-QDC composite make them advantageous over other optical nanoprobes (even with better detection limits) – which usually suffered from the drawbacks such as the concern of using toxic heavy metals (affecting the environmental sustainability), and complicated and costly procedure.43-44 Furthermore, the selectivity of the luminescence of BSA-QDC nanocomposite, towards sensing of trypsin, was tested using different interfering substances (such as L-Lysine, L-Cysteine, glucose, glycine, glutathione, urea, lysozyme, GOx, Ca2+, Zn2+, K+) with higher concentration in comparison to the


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maximum concentration of trypsin used here. No noticeable decrease in emission intensity (at 500 nm) of the BSA-QDC nanocomposite, followed by addition of the mentioned interfering substances (Figure 3C), was observed. This clearly demonstrated the high selectivity of BSAQDC for trypsin sensing in the presence of the mentioned interfering substances. On the other hand, no significant quenching in terms of emission intensity was observed when as synthesized QDC was treated with trypsin (Figure S11, Supporting Information). This control experiment clearly supported that the overserved quenching in emission intensity of BSA-QDC nanocomposite – following trypsin addition – is clearly due to the trypsin induced BSA protein cleavage. This is further supported from similar earlier studies.16-17 Thus, it may be concluded here that the trypsin assisted cleavage of BSA plays an important role for quenching the emission intensity of BSA-QDC nanocomposite (at 500 nm) – following the cleavage and removal of BSA from the surface of QDC. This result also indicated the presence of BSA on the surface of QDC and thus to the successful formation of BSA-QDC nanocomposite. The presented results clearly indicated the applicability of BSA-QDC as a new optical sensing platform for the detection of trypsin.


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3.4. White Light Emission from BSA-dual emitting QDC (ZnQ2 complexed Mn2+ doped ZnS Qdots)

Figure 4. (A) Emission spectra (ex-330 nm) and (B) corresponding chromaticity coordinates in CIE diagram of (i) Mn2+ doped ZnS Qdots (ii) QDC (composed of Mn2+ doped ZnS Qdots and ZnQ2 complex) and (iii) BSA - dual emitting QDC nanocomposite. (C) Digital photograph (under 330 nm light captured with a spectrofluorimeter) of the aqueous dispersion BSA-dual emitting QDC (composed of Mn2+ doped ZnS Qdots and ZnQ2 complex) nanocomposite. (D) Digital photographs (under 330 nm light captured with a spectrofluorimeter) (E) emission spectra (ex330 nm) and (F) corresponding chromaticity coordinates in CIE diagram of the solid BSA-dual emitting QDC nanocomposite. The current strategy of coupling BSA protein to the surface of QDC (followed by purification with the help of centrifugation) could be useful for generating white light emission for a single wavelength excitation. This could be possible when BSA protein coupled with a dual emitting QDC – composed of orange emitting Mn2+-doped ZnS Qdot (with 2.84% the


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concentration of Mn2+ (in mol%))30 and green emitting surface ZnQ2 complex. It is to be mentioned here that the generation of white light from a dual emitting QDC, with excellent chromaticity (0.33, 0.33), high CRI (>80) and CCT (near to 6000-6500 K) values close to the bright day light emitter, 21-31

was difficult to achieve with the help of varying the excitation wavelengths or the concertation

of the complexing agent (which was used during the synthesis of dual emitting QDC).34 This is may be due to the lack of the blue emissive part, in dual emitting QDC, which limited to make them a suitable WLE material, with properties close to day bright light. However, using the protein coupling strategy, the generation of white light emission could be possible even from the same dual emitting QDC. It is to be mentioned here that the use of BSA protein as a surface modifying agent of dual emitting QDC make the process of fabrication of WLE material easier and simpler compared to the earlier reported process,31 where BSA was used as a template for fabricating WLE material followed by incorporating red emitting gold nanoclusters and green emitting ZnQ2 attached ZnS Qdots and having a chance of nonradiative energy transfer between two nanocrystals. Thus, the use one nanocrystal (for example, Mn2+ doped ZnS Qdot) followed by two consecutive surface modification (firstly with ZnQ2 complex and then with BSA protein) may will bring newer avenues for the fabrication of a WLE material. Experimentally, when 1.5 M of BSA treated with a dual emitting QDC (having absorbance of 0.12 at 330 nm) – which exhibited two emissions at 595 (due to dopant of Mn2+-ZnS Qdot)30, 34 and 500 nm (due to HOMO-LUMO transition of ZnQ2 complex) ,30-34 respectively – the emission in the range of 400-550 nm (for a single wavelength excitation wavelength of 330 nm) was broaden (with an enhancement in emission intensity (Figure 4A)). This is similar as demonstrated in the case of the BSA treated QDC (composed of ZnS Qdot and ZnQ2 complex). While no significant change in emission intensity of as such Mn2+-ZnS Qdot (having emission at 595 nm), following addition of BSA, was observed (Figure S12, Supporting


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Information). Thus, the broadening and enhancement of the emission in the range of 400-550 nm may be due to the interaction of the protein with the ZnQ2 complex present in the dual emitting QDC. Further, the BSA treated dual emitting QDC exhibited chromaticity of (0.30, 0.35), CRI of 86 and CCT of 5646 K in liquid phase (Figure 4B and Table S3, Supporting Information). This clearly indicated the bright cool white light nature of the fabricated BSA- dual emitting QDC nanocomposite. On the other hand, only QDC and Mn doped ZnS Qdots (in liquid phase) exhibited chromaticities of (0.44, 0.41) and (0.55, 0.39), CRI of 76 and 35 and CCT of 3062 and 1677 K respectively (Figure 4B and Table S3, Supporting Information). Importantly, the digital photographs of BSA-dual emitting QDC nanocomposite (captured through a spectrofluorimeter under 330 nm light) clearly showed their white light emitting capability (Figure 4C). Thus the protein enhanced luminescence of surface ZnQ2 complex in the blue-green emitting zone (due to activation of their surface states followed by gaining structural rigidity in presence of BSA) – in addition to orange emission (due to 4T1-6A1 transition of Mn2+ doped ZnS Qdot) – present in a single component BSA-QDC nanocomposite helped them to emit white light for a single wavelength of excitation at 330 nm. Additionally, the luminescence quantum yield (QY) of the WLE BSA-dual emitting QDC was measured to be 4.03%, at the excitation wavelength of 330 nm and using quinine sulphate as standard (Table S1, Supporting Information). It is to be mentioned here that the low QY of the WLE QDC may be due to low temperature and aqueous based fabrication process, which is followed here in. Furthermore, the thermal stability of the WLE BSAdual emitting QDC was tested - in terms of their emission spectrum and color chromaticity coordinates - followed by sequentially increasing their temperature from 25oC (room temperature) to 45oC. Interestingly, no significant change was observed in terms of the color chromaticity of WLE QDC – following heating at different temperature in the range of 25-45 oC (Figure S13,


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Supporting Information). Interestingly, solid BSA-dual emitting QDC (followed by casting the liquid dispersion of QDC on a quartz substrate and dried at room temperature) exhibited white light, with emission spectrum similar to that of their liquid phase, for a single wavelength excitation of 330 nm (Figure 4D). Notably, color chromaticity coordinates of (0.29, 0.35), CRI of 80 and CCT of 7377 K were observed for the solid BSA-dual emitting QDC (Figure 4E-F and Table S3, Supporting Information). This clearly indicated the bright cool white light nature BSAdual emitting QDC nanocomposite in their solid form and thus may have their future use in the application of fabricating white LEDs. Thus, the use of nontoxic photostable highly green luminescent BSA coupled QDC (comprised of BSA and ZnQ2 attached ZnS Qdots) as a bioimaging agent to image cervical cancerous HeLa cells and as a low-cost environment-friendly optical sensor for the detection of an enzyme, with detection limit of 0.06 M. Importantly, the same BSA coupling strategy have extended their use to fabricate a bright single component WLE material, followed by interaction of BSA with a dual emitting QDC (consisted of green emitting surface ZnQ2 complex and orange emitting Mn2+- doped ZnS Qdot). 4. Conclusion In conclusion, a new, cost-effective and sustainable surface modification strategy of coupling the bovine serum albumin (BSA) protein with a presynthesized quantum dot complex (QDC: composed of surface Zn-quinolate (ZnQ2) complex and Zn-chalcogenide Qdots (here both undoped ZnS Qdots or Mn2+ doped ZnS Qdots were used separately) is reported herein. The BSA coupled QDC (comprised of ZnQ2 complex and undoped ZnS Qdots) exhibited enhanced optical features (such as high quantum yield and emission life time) and better solubility and stability, compared to only QDC. The structural rigidity and activation of surface states of the ZnQ2 complex


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(present in QDC) followed by Langmuir type binding isotherm with BSA may be the reason behind the enhanced luminescence properties of the biocompatible photostable BSA-QDC nanocomposite. Importantly, the highly luminescent photostable nontoxic BSA-QDC nanocomposite (having BSA, surface ZnQ2 and ZnS Qdots as their components) have been effectively demonstrated their use as a green bioimaging agent to image human cervical cancer HeLa cells and also to detect trypsin (with a detection limit of 0.06 M) in the presence of the mentioined interferring substances. Interestingly, the coupling of BSA with a dual emitting QDC (composed of orange emitting Mn2+ doped ZnS Qdot and green emitting surface ZnQ2 complex) resulted the formation of a single component bright cool white light emitting material, having chromaticity coordinates of (0.33, 0.35) and (0.29, 0.35), CRI of 86 and 80 and CCT of 5646 and 7377 K in their liquid and solid phases respectively. Therfore, the presented surface modification strategy helped to fabricate a versatile multifunational nanocomposite – having potential applications in bioimaging, sensing and white light generation. Furthermore, the presented study endows a new surface chemistry – involving protein, inorganic complex and quantum dot – which will bring a newer dimension for fabricating new optical nanocomposites for the advacement of technological applications of multifunctional nanometarials in energy, environement and heathcare. ASSOCIATED CONTENT Supporting Information. Supporting Information (Figure S1- S13 and Table S1-S3) is available from the ACS website. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] ORCID ID: 0000-0002-9150-6212 Present Addresses. Centre for Nano and Material Sciences, JAIN (Deemed to be University), Jain Global Campus, Bangalore 562112, India. Author Contributions. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources. The authors thank the Department of Science & Technology (DST/INSPIRE/04/2017/001910; IFA17-CH287) Government of India for providing fellowship.

Notes. Dedicated to Prof. Arun Chattopadhyay ACKNOWLEDGMENT. Assistance from CIF, IIT Guwahati, Mihir Manna, Milan Mahadani, Ashim Malakar, and Ayan Pal is acknowledged here. This paper is dedicated to Prof. Arun Chattopadhyay for his constant help, support and advices. REFERENCES


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Enhanced Luminescence of a Quantum Dot Complex Following

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