Article pubs.acs.org/JPCC
Impact of Redox-Active Molecules on the Fluorescence of PolymerWrapped Carbon Nanotubes Elena Polo and Sebastian Kruss* Institute of Physical Chemistry, Department of Chemistry, Göttingen University Germany, Tammannstrasse 6, 37077 Göttingen, Germany S Supporting Information *
ABSTRACT: The near-infrared (nIR) fluorescence of polymer-wrapped single-walled carbon nanotubes (SWCNTs) is very sensitive to the local chemical environment. It has been shown that certain small reducing molecules can increase the fluorescence of SWCNTs. However, so far the role of the polymer around the SWCNT as well as the mechanism is not understood. Here, we investigated how reducing and oxidizing small molecules affect the nIR fluorescence of polymer-wrapped SWCNTs. Our results show that the polymer plays an essential role. Reducing molecules such as ascorbic acid, epinephrine, and trolox increased the nIR fluorescence up to 250% but only if SWCNTs were suspended in negatively charged polymers such as DNA or poly(acrylic acid) (PAA). In comparison, phospholipid−poly(ethylene glycol) wrapped SWCNTs did not respond at all while positively charged polyallylamine-wrapped SWCNTs were quenched. Oxidized equivalents such as dehydroascorbic acid did not show a clear tendency to quench or increase fluorescence. Only riboflavin with an intermediate oxidation potential and light absorption in the visible range quenched all polymerwrapped SWCNTs. In general, polymer-wrapped SWCNTs that responded to reducing molecules (e.g., +141%, ascorbic acid) also responded to oxidizing molecules (e.g., −81%, riboflavin). Nevertheless, several reducing molecules showed only a small fluorescence increase (NADH, +21%) or even a decrease (glutathione, −14%), which highlights that the redox potential alone cannot explain fluorescence changes. Furthermore, we show that neither changes of absorption cross sections, scavenging of reactive oxygen species (ROS), nor free surface areas on SWCNTs explain the observed patterns. However, results are in agreement either with a redox reaction of the polymer or conformational changes of the polymer that change fluorescence decay routes. In summary, we show that the polymer around SWCNTs governs how redox-active molecules change nIR fluorescence (quantum yield) of SWCNTs. Molecules with a low redox potential ( 0, red; ΔI/I0 ≈ 0, white; ΔI/I0 < 0, blue). The results that are displayed in Figure 3c can be analyzed from a polymer perspective:
included ascorbic acid/oxidized ascorbic acid, nicotinamide adenine dinucleotide (NAD)/reduced nicotinamide adenine dinucleotide (NADH), dopamine, (−)-epinephrine, 6-hydroxy2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox), cysteine/cystin, glutathione/oxidized glutathione, and riboflavin. On the other side we wanted to understand how the polymer around the SWCNTs modulates the nIR fluorescence. Thus, we suspended SWCNTs (CoMoCAT) in different DNA sequences, phospholipid−poly(ethylene glycol) (PEG), poly(acrylic acid) (PAA), and polyallylamine (PAH) (Figure 2b). DNA is frequently used to suspend SWCNTs, and because of the phosphate groups, it is negatively charged.25 PAA was chosen because the carboxylic acid groups are negatively charged in physiological buffer. PAH was chosen because it should be positively charged in physiological buffer. Phospholipid−poly(ethylene glycol)s are thought to be rather neutral and inert due to the long poly(ethylene glycol) chains. All of the SWCNT/polymer combinations were characterized by UV−vis−nIR absorption spectroscopy (Supporting Information Figure S1). Those spectra were used to prepare samples of identical SWCNT concentrations and guaranteed absence of aggregation, which would quench fluorescence. Samples were then diluted in phosphate buffered saline (PBS) at pH 7.4. They were analyzed in a Raman setup (785 nm), and the emission spectra between 800 and 1050 nm were recorded. Therefore, our spectra contained both fluorescence and Raman features (Figure 3a,b). Fluorescence spectra were collected C
DOI: 10.1021/acs.jpcc.5b12183 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 3. Fluorescence spectra of SWCNT/polymer complexes and responses to redox-active molecules. (a) Fluorescence/Raman spectrum of SWCNT/(GT)15−ssDNA complexes before (black) and after (red) addition of ascorbic acid (100 μM). Note that the spectra were collected in a Raman spectrometer (785 nm excitation), and therefore SWCNT Raman features such as the G and G′ peak appear. (n, m) indices indicate SWCNT species of different chirality. (b) Same SWCNT/(GT)15−ssDNA complex but addition of riboflavin (100 μM). (c) Normalized fluorescence changes (I − I0)/I0 of different SWCNT/polymer complexes upon addition of different analytes (100 μM) are shown in a color-coded heat map. Red indicates a fluorescence increase and blue a fluorescence decrease. Different SWCNT/polymer conjugates are shown along the x-axis and analyte molecules along the y-axis.
(1) DNA-wrapped SWCNTs increased their fluorescence in the presence of several (reducing) molecules but not all of them. (2) The negatively charged polymer PAA caused a comparable profile. (3) The positively charged polymer PAH showed an inverted response (fluorescence decrease). (4) SWCNTs wrapped with phospholipid−PEG did not respond (only to riboflavin). These results indicate that polymer chemistry is very important for the optical properties of SWCNTs in this context. On the other hand, the responses of the same polymer/ SWCNT complex are different for different small molecules. We observed the strongest fluorescence changes for ascorbic acid, epinephrine, dopamine, and trolox (141 ± 39%, 114 ± 16%, 84 ± 8%, and 151 ± 22% for SWCNT/(GT)15−ssDNA complexes). Other reducing agents showed only small changes such as NADH or glutathione (21 ± 5%, −14 ± 3%). Compared to their oxidized equivalents, only ascorbic acid showed a consistent fluorescent increase that was stronger than that for oxidized ascorbic acid (141 ± 39% for ascorbic acid and 15 ± 12% for the oxidized equivalent). For the other pairs NAD/NADH, cysteine/cysteine, and glutathione/oxidized glutathione we did not find a clear trend (−14 ± 2% vs 21 ± 5% for NAD/NADH, 8 ± 7% vs 2 ± 6% for cysteine/cystin, and −14 ± 3% vs −26 ± 3% for glutathione/oxidized glutathione). Surprisingly, even oxidized ascorbic acid increased the fluorescence in certain cases. In contrast, riboflavin decreased the fluorescence of all tested SWCNT/polymer complexes (−16% to −81%). Even phospholipid−PEG/ SWCNT complexes that did not respond to any other reducing
or oxidizing molecule were slightly quenched (−16 ± 2%). We collected a 2D excitation−emission spectrum of riboflavin with and without SWCNT/(GT)15−ssDNA (Figure S6). The intrinsic fluorescence of riboflavin in the visible range between 500 and 580 nm is higher in the presence of SWCNTs. Therefore, it is likely that riboflavin gets brighter due to energy transfer from SWCNTs, which are simultaneously becoming dimer. Riboflavin is a special case because it is the only molecule of our panel (see Figure 2a) that absorbs light in the visible range (absorption maximum 520 nm) and displays fluorescence (see also the absorption spectra in Figure S5). Energy transfer from perylene-containing surfactants to SWCNTs has been described, but energy transfer from riboflavin to SWCNTs acts in the opposite way.26 We ruled out that the analytes are enhancing/decreasing the absorption cross section by collecting absorption spectra (Figure S5) before and after adding analytes. Absorption spectra did not change, and therefore it is not likely that SWCNTs are directly reduced or oxidized by the analytes because it is known that changes of the free carrier concentration governs absorption and fluorescence spectra.23 Impact of Chirality on Fluorescence Modulation. SWCNTs of different chirality have different redox potentials.27 The oxidation potential of nanotubes decreases with increasing diameter.21,27 If electrons were directly transferred between analyte and SWCNT, we would therefore expect a chirality dependence. However, fluorescence changes of different SWCNT/polymer complexes for different SWCNT chiralities (6,4), (9,1), (8,3), D
DOI: 10.1021/acs.jpcc.5b12183 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 4. Fluorescence responses of different SWCNT chiralities. (a−d) Heat plots show responses of different SWCNT/polymer complexes to redox-active molecules (red = increase, blue = decrease) The x-axis displays different SWCNT chiralities in the sample, which are labeled with the chiral index (n, m). The panels correspond to different polymer wrappings: a, (GT)15; b, (AT)15; c, PL-PEG1.5; d, PAA.
SWCNT in buffer.9 If ROS-scavenging played a role in the mechanism, addition of a scavenger should (a) strongly affect the fluorescence and (b) abolish the response to other molecules (e.g., ascorbic acid). However, we observed very small changes of SWCNT fluorescence when adding those compounds (
