Boron Precursor-Dependent Evolution of ... - ACS Publications

13 downloads 0 Views 5MB Size Report
Dec 26, 2016 - Department of Chemistry, Furman University, Greenville, South Carolina 29613, United States. §. Theoretical Chemistry Section, Chemistry ...
Article pubs.acs.org/Langmuir

Boron Precursor-Dependent Evolution of Differently Emitting Carbon Dots Jayasmita Jana,† Mainak Ganguly,‡ Kuttay R. S. Chandrakumar,§ Gowravaram Mohan Rao,∥ and Tarasankar Pal*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Department of Chemistry, Furman University, Greenville, South Carolina 29613, United States § Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India ∥ Department of Instrumentation, Indian Institute of Science, Bangalore 560 012, India ‡

S Supporting Information *

ABSTRACT: Attention has been directed toward electrondeficient boron doping in carbon dots (CDs) with the expectation of revealing new photophysical aspects in accordance with varying amounts of boron content. It has been emphatically shown that boron uptake in CDs varies with different boron precursors evolving altered emissive CDs. Boron doping in CDs causes definite surface defect due to the generation of electron-deficient states. Modified hydrothermal treatment of a mixture of ascorbic acid (AA) and different boron precursor compounds (borax/boric acid/sodium borate/sodium borohydride) produces different kinds of boron-doped CDs (BCDs). These BCDs ( B4CD > B2CD > B3CD (λem = 460, 453, 452, and 445 nm, respectively). Also the position of peak maxima differs for the CDs. This situation can be explained by considering the density of the doped hetero atom, boron. It may be mentioned that the electron-deficient boron causes the surface defect in CDs and the effect is the generation of electron-deficient states.55 Because in B1CD, a 576

DOI: 10.1021/acs.langmuir.6b04100 Langmuir 2017, 33, 573−584

Article

Langmuir

Figure 3. (A) Optimized geometries of boron- and boron oxide-doped CDs. (B) UV−vis spectrum of boron- and boron oxide-doped CDs (red, light red, black, and gray spheres represent oxygen, boron, carbon, and hydrogen atoms, respectively).

We have studied the fluorescence spectral profile of these CD solutions over two months at room temperature. Virtually, no change in the emission intensity and the emission wavelength is observed, indicating the long-term stability of the assynthesized fluorescent particles (Figure S3). The chemical or molecular structures of the as-synthesized BCDs could not be defined properly. However, FTIR and XPS studies indicate that they possess C−B, CC, C−O, C−H, B−O−C, CO, and −O−/−OH bonds. The DFT studies are performed where the model structures have been considered. The geometry of the model CDs is constructed with 19 benzenoid fused ring structures, without any symmetry constraints. The boron atoms are introduced in the CD framework as boron atoms, and boron oxides (B2O and B2O3) are introduced at the center of the model CD systems. The terminal carbons are saturated with a hydrogen atom, and also

emission peak is due to the presence of the hetero atom, boron. As boron is an electron-deficient atom, an electron-deficient state (V0) is generated above the actual ground state (S0). Thus, the excited electron has to travel a short distance toward the ground state during radiative decay (Figure S4). Hence, the emission peak will be red-shifted. The quantum yields of B1CD, B2CD, B3CD, and B4CD are 5.45, 3.43, 2.13, and 4%, respectively, with respect to quinine sulfate prepared in 0.1 M H2SO4 solution. Lifetime measurement shows that B1CD, B2CD, B3CD, and B4CD have average lifetimes of 16.12, 9.27, 7.1, and 14.7 ns, respectively (Figure S5). A comparative table (Table 1) has been presented for all of these four types of BCDs. However, further investigation is warranted for elaborate explanation. A comparative account for the optical properties of some other BCDs have also been provided in Table S1. The assynthesized BCDs are very stable in terms of emissive property. 577

DOI: 10.1021/acs.langmuir.6b04100 Langmuir 2017, 33, 573−584

Article

Langmuir

carry a partial negative charge, which leads to the formation of a positive charge in the framework, leading to the formation of defects on the hexagonal carbon surface, which proportionately increases with respect to the boron ion concentration. Although in the case of B2O3, boron appeared to have a positive charge, the oxygen atom of B2O3, being more electronegative than boron, becomes a more negatively charged center, which also causes the formation of a positively charged carbon surface. The charge polarization caused by the introduction of the dopant hetero atoms is likely to be one of the important factors for the observed red shift in the UV absorption spectrum. In addition, it can also be noticed from Table 2 that the calculated dipole moments of all compounds substantially increase upon boron doping, and it can also be seen that higher the concentration of boron, higher is the dipole moment. For instance, the change in the dipole moment is almost six times more for the boron oxide systems compared with that of pure carbon systems. In the case of merely boron atom-doped CD, the increase in the dipole moment, from 2.497 (CD) to 3.754 (4B-CD) debye, is not very significant. The highest dipole moment is observed in the case of 2B2O3-CD, which is 16.07 debye. The substantially enhanced dipole moments in the case of boron-doped systems lucidly demonstrate the existence of charge polarization, which can have a dramatic influence on the UV absorption behavior of these BCDs. It should be noted that the dipole moment values appear to be quantitatively very high, which may be due to the usage of medium level basis sets. Nevertheless, the observed dipole moment variations can be perceived qualitatively. The summary of the theoretical calculations apparently suggests that the electronic transitions and the associated red shifts in the absorption spectrum can be correlated with the concentration of boron atoms, and these effects are mainly caused by the huge charge polarization within the carbon surface, leading to the formation of surface defects. Here, one point should be mentioned that the feed boron ratio has an effect on the surface and optical properties of BCDs. Although the initial concentrations of boron precursor compounds are the same, the atomic percentage of boron is different in the compounds. Also boron is being doped as a boron oxide moiety. The extent of doping increases as the boron content of the precursor compounds increases. However, from XPS and FTIR studies, it is found that in all four BCDs, C−O, C−B, and B−O bonds are commonly present, but the optical properties differ greatly. As optical properties, especially fluorescence, largely depend on the electronic states of the substrate, the difference in the fluorescence intensity and the peak position indicates that the BCDs are in a different electronic environment. The BCDs become different because of the varied amounts of doped boron that causes different charge polarizations on the carbon surface leading to surface defects. From the experimental findings, we conclude that B1CD is the most effective and pronounced boron-doped product among the as-synthesized CDs. Henceforth, we choose to use B1CD for further investigation. It is also worth noting that B1CD exhibits the highest quantum yield (Φ) with highest boron content. Emission profile of B1CD is found to be pHdependent. In acidic medium, the emission peak shows a red shifting of ∼15 from 460 nm. Again, with the increase in pH of the medium, blue shift occurs and approaches 460 nm (Figure S6A). However, the intensity of emission does not follow any proper order. From pH = 1, the intensity gradually increases until pH = 9, decreasing again (Figure S6B). This shift can be explained by the reversible binding of protons to the emissive

four OH groups are introduced at the terminal position (Figure 3A). This model qualifies for the highest stability. On the basis of this theoretical model, further studies are performed, which agreed well with the experimental results. Simultaneous theoretical calculations also justify the optical behavior of the as-synthesized CDs. The optimized geometry of the pure CD, as obtained using the DFT-based BP exchange−correlation function, is shown in Figure 3A. The calculated dipole moment, charge on boron atom and oscillator strengths, maximum wavelengths (λ) for CD, boron, and boron oxide-doped systems are presented in Table 2. The absorption spectra Table 2. Dominant Optical Transition Characters for the Lowest Excitation States of the CDs with and without Boron Dopants, Calculated Using TDDFT-Based BHHLYP/def2SVP system

oscillator strength

maximum wavelength λ (nm)

dipole moment (debye)

All-H-4OH 2B-4OH 4B-4OH

0.92 0.33 0.30

340.20 504.70 593.60

2.49 3.56 3.75

B2O-4OH

0.14

918.20

5.31

2B2O-4OH

0.12

861.00

10.19

B2O3-4OH 2B2O3-4OH

0.12 0.07

553.40 701.90

9.52 16.07

charge on boron (au) −0.23; −0.11 −0.15; −0.11; −0.03; −0.09 −0.23; −0.15; −0.09 −0.21; −0.11; −0.21; −0.12 −0.06; −0.17 0.06; −0.14; −0.05; −0.13

calculated using the TDDFT method for the CD with four hydroxyl groups, boron, and boron oxide-doped CDs are shown in Figure 3B. For the undoped CD at equilibrium geometry, the spectra exhibit a single dominant peak at 340 nm and a small satellite peak at 473 nm, corresponding to the excitation of HOMO-1 and HOMO to LUMO of the complex, respectively. The dominant peak undergoes red shifts systematically from 340 to 918 nm as the boron is gradually introduced in the form of boron atoms and boron oxides in the CD. It is also worthwhile to note that although such red shifts are observed in the case of boron-doped compounds, the maximum shift is exhibited by the system with higher amounts of boron. For instance, in the case of B2CD and B4CD, the observed absorption wavelength maximum is 504.7 and 593.6 nm, respectively; similarly for B2O-CD and 2-B2O-CD, it is 918 and 842 nm and for the case of B2O3-CD and 2-B2O3-CD, it is 553 and 701 nm, respectively. Interestingly, the experimentally observed red shift at higher wavelength is at 460 nm for the case of B1CD and for other cases, the variation in the red shift is only within 15 nm. The theoretically predicted results qualitatively demonstrate that the increase in the concentration of boron causes further shift in the absorption peak. The observed red shift in the absorption wavelength presumably arises due to the introduction of boron atoms in the CD. In the case of pure CD, there are sp2 carbon atoms with delocalized electrons within the fused aromatic ring, and introduction of any hetero atom such as boron or boron oxides can induce the formation of surface defects because of the charge polarization or creation of possible electron−hole pairs. Because boron complexes are electron-deficient species, it is expected that boron atoms/groups can accept electrons from the electronrich carbon centers. Evidently, while analyzing the charge population on boron atoms, it can be seen that boron atoms 578

DOI: 10.1021/acs.langmuir.6b04100 Langmuir 2017, 33, 573−584

Article

Langmuir

Figure 4. (A) Fluorescence spectral profile and (B) bar diagram of B1CD in the presence of different metal salts in aqueous solution. Control solution is B1CD solution without any metal salt. I0 = fluorescence intensity of B1CD without metal and I = fluorescence intensity of B1CD in the presence of metals. (C) Fluorescence spectra of B1CD at different concentrations of Fe(III). (D) Stern−Volmer plot regarding quenching of the fluorescence of B1CD in the presence of Fe(III) at different concentrations. (E) and (F) Two linearly fitted plots. (G) Interference study, fluorescence spectra of B1CD solution in the simultaneous presence of Fe(III) and other metals at equimolar concentration. Condition: [B1CD] = 0.3 mL, [metal] = 0.0003 M, pH = 9 and λex = 365 nm.

When Fe(III) is added, the fluorescence is quenched greatly (Figure 4). This quenched solution is termed as Fe(III)B1CD. Gradual quenching of the fluorescence of B1CD with the increased concentration of Fe(III) ion paves the way for Fe(III) sensing in aqueous medium. Using this method, we can detect Fe(III) down to 3.1 nM level (Figure 4). This is much lower than the permissible limit of Fe(III) in water, that is, 0.3 mg/L or ∼5.357 μM, as prescribed by the World Health Organization. The relative fluorescence intensity varies linearly over two concentration ranges. The correlation coefficient (R2) is 0.989 for the 3−1000 nM concentration range and 0.985 for the 3−300 μM range. Thus, we are able to measure the Fe(III) for a wide range of concentrations. Also the interference study

sites at low pH. Low pH protonation shows a blue shift, whereas increase in pH deprotonation causes a red shift.56 However, at high pH, there may be noncovalent molecular interactions through the hydrogen bond between −OH groups, causing quenching of fluorescence.57 Hence, by varying the pH, the fluorescence can be tuned. The experimental pH has been set to 9. B1CD contains oxygenated functional groups on the surface, which can bind metals easily. That encouraged us to use B1CD as the metal detection probe. Different metal salts [Cd(II), Co(III), Cu(II), Cr(III), Fe(II), Fe(III), Hg(II), Mg(II), Ni(II), Pb(II), and Zn(II)] are added to the B1CD solution. The fluorescence is measured within 30 min of metal addition. 579

DOI: 10.1021/acs.langmuir.6b04100 Langmuir 2017, 33, 573−584

Article

Langmuir

quencher shortens the excited state lifetime. This provides more weightage to the emission process from only relaxed excited states (which are higher in energy than totally solventrelaxed states). The smaller size of Fe(III) cation helps in such chelation. TEM image (Figure 5B) shows the aggregated form of Fe(III)B1CD. Also, we have varied salts of Fe(III) to check the quenching phenomenon, and it is found that all salts of Fe(III) exhibit the same quenching effect, confirming that Fe(III) cation is the root cause of quenching. Thus, the quenching effect of Fe(III) is independent of the counter anion. Figure 5C shows the effect of different Fe(III) salts on B1CD. The lifetimes of B1CD and Fe(III)B1CD differ greatly (16.12 and 3.16 ns), so also the UV−vis spectra differ for both the species (Figure 5D). Thus, there lies a possibility of dynamic quenching.24 The quenching efficiency can be fitted to Stern− Volmer equation

indicates that the quenching capability of Fe(III) is not significantly affected when the other cations are present in the solution as interfering agents (Figure 4). Different environmental water samples (drinking water, tap water, and river water) were used to establish the utility of the as-synthesized B1CDs. In these water samples, a certain extent of Fe(III) was spiked using the standard addition method. Different interfering cations of fixed concentrations are added while the concentration of Fe(III) is varied, spiking the water samples. The detected concentration, recovery percentage, and the relative standard deviation (%) of Fe(III) in water samples are listed in Table S2. The recovery percentage of Fe(III) ions in these samples agrees well, justifying the applicability of the present method. The quenching B1CD solution in the presence of Fe(III) can be explained by considering the facile binding of Fe(III) with the surface groups of B1CD particles. Fe(III) has an inherent and strong affinity toward −O−, as reported by Zhang et al.58 The quenching of B1CD solution in the presence of Fe(III) can be explained by considering the facile binding of Fe(III) with the surface groups of B1CD particles (zeta potential is −2.12 mV). It is assumed that upon the addition of Fe(III) ion into a solution of B1CD, there happens a fast electron transfer reaction (Figure 5A). Here, the electronic interaction of

I0/I = 1 + KSV [Q]

where I0 and I are the fluorescence intensities of B1CD without and with Fe(III) ion, respectively, Ksv is the Stern−Volmer constant, and [Q] is the concentration of the quencher, Fe(III). The I0/I versus [Q] plot (Figure 4) shows linearity over a wide range (3−1000 nM and 3−300 μM with R2 = 0.989 and 0.985, respectively), indicating static quenching. Hence, it is concluded that both static and dynamic quenching occur during the quenching of fluorescence of B1CD. It is worth mentioning that B2CD, B3CD, and B4CD show comparatively less pronounced fluorescence quenching in the presence of Fe(III) ions, and the measured limit of detection (LOD) values are 12.9, 15.4, and 7.5 nM for B2CD, B3CD, and B4CD, respectively (Figure S7). On the other hand, B1CD is chosen as the most effective probe because of its highest sensitivity for Fe(III) quantification with an LOD value of 3.1 nM. It is also seen that undoped CDs (CDa, product obtained after MHT of AA) undergo fluorescence quenching in the presence of Fe(III). Fe(III) can be sensed in solution using CDa down to 21 nM level with a linear range of detection of 1−300 μM level (R2 = 0.984) (Figure S8). It is proposed that the quenching is prompted by the chelation of Fe(III). In fact, facile chelation occurs because of the presence of −O−/ hydroxyl groups at the edges of CDs, as mentioned earlier. Higher fluorescent intensity for BCDs causes much effective and more sensitive detection of Fe(III) in water samples. A comparative table (Table S3) has been accounted to show the efficiency of our prescribed method. However, Fe(II) interferes to some extent in Fe(III) sensing. To eliminate this interference, we have serendipitously found an effective reagent, fructose. Upon the addition of fructose to Fe(II)B1CD [Fe(II) induced quenched B1CD solutions] and Fe(III)B1CD individually, the two solutions showed different behaviors, whereas fluorescence intensity was measured within 10 min of fructose addition. Interestingly, it was observed that the fluorescence of Fe(III)B1CD is selectively increased with fructose, whereas that for Fe(II)B1CD solution remained quenched/unaltered (Figure 6A). It is well-established that Fe(III) has a tendency to bind with D-fructose. Tonković59 showed that the Fe(III)−fructose complex formation is evident at pH = 11. We have observed that as the pH is lowered down (pH = 9), there occurs a competitive interaction of Fe(III) with fructose and B1CD, whereas the latter remains present in solution. At this pH condition (