Photoinduced Electron Transfer in Carbon Dots with Long-Wavelength

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C: Physical Processes in Nanomaterials and Nanostructures

Photoinduced Electron Transfer in Carbon Dots with Long Wavelength Photoluminescence Keenan J. Mintz, Brenda Guerrero, and Roger M. Leblanc J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06868 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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The Journal of Physical Chemistry

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Photoinduced Electron Transfer in Carbon Dots with Long Wavelength Photoluminescence Keenan J. Mintza, Brenda Guerreroa, and Roger M. Leblanca* a. Department of Chemistry, University of Miami, Coral Gables, Florida 33146 (USA). *Corresponding Author: [email protected] Abstract Carbon dots have often been studied to investigate their unique optical properties such as excitation wavelength-independence emission. Carbon dots have also been shown to undergo electron transfer in different situations. This study endeavors to investigate the properties of carbon dots photoluminescence and electron transfer. Herein, the preparation and characterization of carbon dots which exhibit long wavelength photoluminescence has been reported. These carbon dots exhibit quenching when exposed to metal ions in proportion to the reduction potential of the metal, which experimental evidence has shown for the first time. This property of metal ion reduction potential-dependent quenching has been studied to show the collisional electron transfer from amine groups in carbon dots to the metal ions. Therefore, the photoluminescence in these carbon dots are directly related to organic functional groups on the surface of the carbon dots. Introduction Carbon dots (CDs) are a class of carbon nanomaterials discovered in 2004 which have gained interest over the last decade and a half for their unique properties, including low toxicity/biocompatibility, small size (< 10 nm), and excitation wavelength-dependent emission.1-5 This last point has drawn considerable interest, especially since excitation wavelengthindependent emission has begun to be recently reported for CDs.6-7 The photoluminescence (PL) mechanism of CDs is under a great amount of scrutiny at the moment. Many theories have been proposed including: PL resulting from quantum confinement as in traditional quantum dots, the synthesis of fluorophores, and surface state energy traps.8-11 Work is needed in the area of CDs PL mechanism to clearly define the pathway by which the photoluminescence of CDs occurs.

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One recent research goal for CDs has been long wavelength PL, particularly for bioimaging applications.12-13 This study presents a novel pathway to the goal of long wavelength PL. Evans blue (structure 1) is a dye commonly used in biological studies due to its affinity for serum albumin proteins.14-15 This interaction has been shown to occur through electrostatic and hydrophobic interactions in a serum albumin binding pocket.16-17 It has also been shown that approximately 14 Evans blue molecules can fit in this binding pocket through the supramolecular assembly of the Evans blue monomers in solution.18-19 Evans blue also possesses long wavelength absorption and fluorescence (ca. 600 and 660 nm, respectively). Proteins, such as bovine serum albumin (BSA), have been used many times to form CDs20-21, but the complex of Evans blue with a serum albumin protein such as BSA could be used to obtain CDs with long wavelength PL. Long wavelength CDs also provide the advantage of having a larger range of wavelengths from which to select the desired optical properties. This could manifest in the selection of a wavelength for energy transfer or bio-imaging, and it also provides the potential to study the nature of CDs optical properties at more wavelengths.

Structure 1: Structure of Evans blue CDs have commonly been used as a tool to detect metal ions through the quenching of the CDs PL by the metals.22-24 This property has many applications in everyday life as there are many reasons the detection and quantification of metal in a material or solution are desirable.25-27 The usual means of establishing a quantitative relationship, between the PL response of CDs (to metal ions) and the metal ion concentration, is through a Stern-Vølmer plot.28-30 The Stern-Vølmer equation was developed by Otto Stern and Max Vølmer and can be presented as seen in equation 1.31


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3 F0 F

= 1 + KSV[Q]

Equation 1

F0 describes the signal intensity (this study uses the peak intensity of PL) when no quencher (Q) is present. F indicates the signal intensity when Q is present. A plot of F0/F vs. [Q] (concentration of quencher) should give a linear plot with the slope of the line equaling the Stern-Vølmer constant (KSV). A Stern-Vølmer plot can be used to determine the type of quenching occurring by changing parameters, such as temperature and viscosity of the solvent used, and observing the changes in the slope of the plot.31 There has been some study regarding electron transfer in CDs.22, 28, 32-37 These studies have been regarding electron transfer to or from a specific organic molecule, free and covalently bound (or electrostatic association), or metal ion.28, 38-46 The transformation of functional groups upon electron transfer to or from CDs is unclear though.47 Additionally, the interaction between CDs and metal cations has been discussed, with various theories proposed (e.g. charge transfer complexes).48-49 However, this study endeavors to provide an analysis of CDs interaction with several metal cations and to provide an explanation for the commonly reported quenching of CDs PL by metals. Electron transfer is often modeled according the Rehm-Weller equation (equation 2), where the Gibbs free energy of the electron transfer process is estimated based on the oxidation potential of the donor (ED/D+), the reduction potential of the acceptor (EA/A-), the excited state energy of the donor (ES), and a coulombic term (C) which accounts for the interaction between donor/acceptor based on their charge.50 ∆Get = ED/D + ― EA/A ― ― ES ―C

Equation 2

The inclusion of the excited state energy of the donor (ES) makes it clear that this equation can only be used to model photoinduced electron transfer (PET) and cannot be used for electron transfer that occurs in the ground state or charge transfer complexes. As previously discussed, many studies have been conducted regarding CDs sensing applications. This study’s focus is the mechanism of this quenching. To the best of our knowledge this study represents the first report of a linear trend between the reduction potential of metal ions and their quenching ability. Based on this trend the electron transfer from CDs to metal ions’ valence d-orbitals was examined and a proposed mechanism for the metal-CDs interaction is



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presented. Additionally, this model was also used to provide insight into the PL mechanism of CDs. Materials Bovine serum albumin (fraction V), NaCl, and Pb(NO3)2 were obtained from MP biomedicals. Evans blue (>75%), Rhodamine B (>99%), MnCl2, SnCl2·2H2O, CsCl, CoCl2·6H2O, NiCl2·6H2O, CuCl2·2H2O, and FeSO4·7H2O were purchased from Sigma-Aldrich. CdCl2·3H2O, CaCl2, and Thermo Scientific Snakeskin Dialysis Tubing (MWCO=3500 Da) were bought from Thermo Scientific. MgSO4·7H2O was purchased from Bio Chemika Int. ZnCl2 was aquired from Alfa Aesar. FeCl3 was bought from Acros Organics. The deionized water used had a resistivity of 18.2 MΩ·cm and was obtained from an Elga PureLab Ultra water purifier. UV-vis characterization was performed on an Agilent Cary 100 UV-vis spectrophotometer. Fluorescence spectra were obtained from a Fluorolog Horiba Jobin Yvon fluorometer with a slit width of 5 nm for excitation and emission. All optical characterization was obtained with quartz cells with a pathlength of 1 cm. AFM images were obtained from an Agilent 5420 atomic force microscope with a silicon tip applied with a force of 3 N/m. TEM images were acquired using a JEOL 1200X TEM. The attenuated total reflection (ATR) infrared spectra were obtained from a Perkin-Elmer FTIR spectrometer with air as a background. The Zeta potential was measured on a Malvern Zetasizer nano-series. Circular dichroism measurements were taken from a Jasco J-810 spectropolarimeter. Methods Carbon Dot Preparation Evans blue (≈31 mg) was mixed with BSA (≈143 mg) in a 15:1 molar ratio in deionized water for 5 minutes to ensure the formation of a complex between the dye and BSA. The mixture was then sealed in a Teflon lined, stainless steel autoclave and heated at 140, 160, 180, and 200 ⁰C for 4 hours. After the reaction was finished, the mixture was sonicated for 30 minutes (42 kHz) and filtered through a 0.2 μm syringe filter to remove large, black particles. Then the filtrate was again sonicated for 30 minutes and the solution was placed in a dialysis bag of MWCO 3500 Da for 5 days, changing the water every 12 hours. After 5 days, the solution was taken and lyophilized


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to obtain the solid sample, which was dark purple in color. Due to the precursor, these carbon dots will be referred to as bovine serum albumin carbon dots (BSA-CDs) Ion Quenching Tests 10 mM metal ion solutions were prepared of Na+, Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Sn2+, Cs+, and Pb2+. These were mixed with a solution of BSA-CDs so that the final concentration of metal ion was 5 mM and of BSA-CDs was 0.1 mg/mL. The fluorescence spectrum (excitation wavelength: 520 nm) for each mixture was recorded and the % quenching was calculated. The excitation wavelength of 520 nm was used to investigate the nature of the long wavelength absorption/PL for the BSA-CDs. Additional tests were performed with different concentrations of ions and carbon dots, as detailed in the results. Results and Discussion Reaction Optimization for BSA-CDs In the reaction of Evans blue and BSA different reaction conditions were tested to see the best preparation method for CDs. The reaction was done with the same procedures for 140, 160, 180, and 200⁰C. The absorption spectra for these reactions (figure S1) shows that the reaction at 140⁰C still possesses Evans blue in solution. To confirm that the changes in absorption spectra are not due to the interaction of Evans blue with BSA, the absorption spectra of the mixture without any heat applied was recorded (figure S2). There is a slight red shift in the peaks of the Evans blue associated with BSA compared to free Evans blue, which can be attributed to the binding pocket of BSA stabilizing Evans blue, but there are no significant changes. The reactions at 160, 180, and 200⁰C do not show any Evans blue, but the reactions at 180 and 200⁰C have a long wavelength absorption peak which is blue shifting and decreasing (with respect to the absorption at shorter wavelengths). The reaction at 160⁰C still possesses long wavelength absorption that is distinct from Evans blue and is a longer wavelength than the longest peaks (or shoulders) in the 180 and 200⁰C reactions. For this reason, 160⁰C was used as the reaction temperature going forward as long wavelength absorption/PL is a desirable property for CDs. Characterization of BSA-CDs



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After the reaction at 160⁰C and dialysis, the solution of the CDs (BSA-CDs), was dark red in concentrated solutions (> 5 mg/mL) and purple upon dilution ( 18000 Da) present in the BSA-CDs as compared to the BSA mass spectrum in the same figure. In figure S8, the large number of peaks around 100 Da indicates the presence of amino acids, but the absence of larger masses in this spectrum, similarly to the MALDI spectrum for BSA-CDs (figure S7), shows that only small peptides remained and must be connected to the BSA-CDs or they would be expelled by dialysis. Ion Quenching of BSA-CDs and Evans Blue To investigate the ability of the BSA-CDs to be quenched by metal ions, several ions were tested with the BSA-CDs and the resultant quenching at 590 nm (excitation at 520 nm) was recorded as seen in figure 4A.

Figure 4: (A) Quenching of BSA-CDs with various metal ions. (B) Plot of ions quenching ability (excitation 520 nm) versus their reduction potential. There appears to be no obvious correlation between any particular ion and its quenching ability. The forces between CDs and metal ions are commonly attributed to be electrostatic in nature, as CDs commonly possess a negative surface charge which can be attracted by the positive charge of the metal ions.55-57 The quenching, however, requires a deeper explanation. A similar trend has been observed for some of these ions, but no determination was made for the difference between all the ions.48 However, when the quenching of the ions from the first row of the d-block of the periodic table (Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II)) were plotted against the ions’ reduction potentials obtained from literature58, an interesting trend was observed as seen in figure 4B. There is a linear trend between the quenching of the BSA-CDs by the ions and the ions


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reduction potential. This trend immediately suggests the mechanism of quenching is through electron transfer from the BSA-CDs to the metal ion. When all the ions tested were plotted against their reduction potential (figure S9), the shape of the plot is similar to what has been observed in multiple Rehm-Weller plots.59 The region of linearity seen in figure 4B is present in this plot (figure S4) and it can be seen that Mn (quenching ≈ 7%) is close to the top, flat portion of the graph. If it is removed from the plot (as in figure S10) the R2 value increases from 0.8981 to 0.9504, which further indicates that electron transfer is occurring. These results corresponds with Huang and coworkers, who attributed the potential of electron transfer to the d-orbital configuration.22 The correspondence of the quenching to the reduction potential confirms this and provides more insight into the electron transfer. The reduction potential corresponds to the stability of the d-orbitals and the energetic favorability of adding an additional electron to the valence dorbital. The metal ion quenching test was repeated at lower ion and BSA-CDs concentration to validate the results (figures S11 and S12) and the results show that the linearity is still observed although the range of linearity has changed (Mn(II) no longer can quench enough to appear linear in these plots). The lower quenching ions, Mn(II) and Zn(II), have moved into the top, flat portion of the graph in figure S9 at the lower concentration. Therefore, the linearity greatly improves when they are discounted at these concentrations (figures S11B,C and S12B,C). Additionally, the quenching between the plots in figures S11 and S12 is very similar, where the only difference is BSA-CDs concentration, suggesting that the quenching is independent of the concentration of BSA-CDs, at the metal concentrations tested. More concentrations of BSA-CDs were tested while keeping the concentration of metal ions constant (figure S13). All concentrations tested once again show very similar slopes and y-intercepts to further confirm the quenching is independent of the concentration of BSA-CDs. The quenching of the Evans blue molecule, free and complexed with BSA, was also examined for the possibilities of similarities with BSA-CDs. No obvious trend could be seen between metal ion and the quenching efficiency (figure S14). Copper still possesses the highest quenching ability, however upon addition of the copper solution to the free Evans blue solution, the color of the solution turned from blue to purple indicating the formation of a ground state complex. This indicates that the mechanism of quenching for copper is different between Evans blue and BSA-CDs. The quenching of Evans blue complexed with BSA was much less than for



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the free Evans blue. This can be explained by the protection of the Evans blue molecules in the BSA binding pocket. The previous tests with BSA-CDs were all conducted upon excitation at 520 nm and measuring the emission at 590 nm. To examine the quenching properties at shorter wavelengths, the same test was performed by exciting at 400 nm and measuring the quenching ability of each ion at an emission wavelength of 490 nm. As can be seen in figure S15, the quenching at shorter wavelengths approximately follows the same trend as for quenching at longer wavelengths (figure S9). However, when the same plot made for excitation at 520 nm (figure 4B) is created for excitation at 400 nm (figure S16), the relationship is shown to be less linear. This can be attributed to the variance of surface states which absorb light close to the UV region.60-61 Multiple environments contribute to the PL of CDs in this range, so the linearity of the plot is decreased. When the excitation wavelength is increased and in a region where functional groups on the surface of CDs do not absorb, the reduction of the metals by CDs can be clearly seen. Stern- Vølmer Analysis of BSA-CDs To investigate this quenching property further, a Stern-Vølmer plot was obtained for copper (II), as it showed the most quenching in the previous tests (ca. 90%). The intensity of the peaks from the photoluminescence spectra was the value taken for F and F0 in all Stern-Vølmer plots. As can be seen in figure 5A and 5B, there is good linearity over a broad range of Cu2+ concentrations (0.5-250 μM) in water.

Figure 5: (A) Fluorescence spectra for BSA-CDs mixed with various concentrations of Cu2+ in water. (B) Stern-Vølmer plot for the quenching of BSA-CDs by Cu2+ in water (blue) and water/glycerol (1:1) (orange).


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From figure 5B and S17, it can be seen that the slope for the solvent with increased viscosity decreases by about half as compared to the plot with water. This decrease in slope indicates that the mechanism of quenching is dynamic (collisional) quenching, since the slope of the plot (KSV) is proportional to the inverse of the viscosity (or 1/η) according to the Stern- Vølmer equation. Based on these plots, the mechanism for quenching is proposed as follows. Before excitation, the CDs and ions are moving through solution randomly. When the CDs are excited they have the possibility of colliding with a metal ion. At this point electron transfer is possible, but the degree to which this transfer will occur depends on the tendency of the metal ion to accept an electron (reduction potential). If electron transfer occurs, then the CDs will relax non-radiatively and the photoluminescence will be quenched. Nickel (II) and cobalt (II) were also analyzed with Stern-Vølmer plots to see if the same trend was observed (figure 6).

Figure 6: (A) Fluorescence spectra for BSA-CDs mixed with various concentrations of Ni2+ in water. (B) Stern-Vølmer plot for the quenching of BSA-CDs by Ni2+ in water (blue) and water/glycerol (1:1) (orange). (C) Fluorescence spectra for BSA-CDs mixed with various concentrations of Co2+ in water. (D) Stern-Vølmer plot for the quenching of BSA-CDs by Co2+ in water (blue) and water/glycerol (1:1) (orange).



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First, it should be noted that the slope of the blue plots (in water) for each ion decreases with decreasing reduction potential (i.e. Co

Photoinduced Electron Transfer in Carbon Dots with Long-Wavelength

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