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Optical Probing of Local pH and Temperature in Complex Fluids with Covalently Functionalized, Semiconducting Carbon Nanotubes Hyejin Kwon, Mijin Kim, Brendan Meany, Yanmei Piao, Lyndsey Rae Powell, and YuHuang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509546d • Publication Date (Web): 28 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015

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The Journal of Physical Chemistry

Optical Probing of Local pH and Temperature in Complex Fluids with Covalently Functionalized, Semiconducting Carbon Nanotubes

Hyejin Kwon‡, Mijin Kim‡, Brendan Meany, Yanmei Piao, Lyndsey R. Powell, YuHuang Wang* Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, United States ‡ These authors contributed equally to this work.

ABSTRACT: We show that local pH can be optically probed through defect photoluminescence from semiconducting carbon nanotubes covalently functionalized with aminoaryl groups. Switching between protonated and de-protonated forms of the amino moiety produces an energy shift in the defect state of the functionalized nanotube by as much as 33 meV in the near infrared region. This unexpected observation enables a new optical pH sensor that features ultra-bright near-infrared II (1.1-1.4 µm) photoluminescence, a sensitivity for pH changes as small as 0.2 pH units over a wide working window that covers the entire physiologic pH range, and potentially molecular resolution. Independent of pH, this nanoprobe can simultaneously act as a nanothermometer by monitoring temperature-modulated changes in photoluminescence intensity, which follows the van’t Hoff equation. This work opens new opportunities for quantitative probing of local pH and temperature changes in complex biological systems.

Keywords: biosensor · defect chemistry · exciton · nanomaterial photophysics · nanoscale probe

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1. INTRODUCTION The concentration of protons in an aqueous solution, measured as pH, and the temperature are important thermodynamic variables that impact processes ranging from material synthesis to cellular dynamics in malignant or tumor cells.1–4 However, it is challenging to quantify these parameters in complex systems such as nanoscale reaction mixtures and complex fluids, especially when high sensitivity, selectivity, and spatial resolution (sub-micrometers) are simultaneously required for remote measurement.5–8 For instance, several approaches have been proposed

to

detect

proton

concentration

or

temperature

response;

among

which

photoluminescence (PL) of organic dyes and synthetic nanoparticles have emerged as promising solutions to in vivo measurement and imaging.9–14 One issue limiting the applications of these optical sensors is the small biological penetration depth of their visible and first near-infrared window (NIR-I, 0.75-0.9µm) PL.6,7 Moreover, when optical sensors based on PL intensity, energy shifts, and emission lifetime and anisotropy are compared to an external standard,6,7 it is difficult to resolve the complex chemical environment of biological systems with high fidelity. Semiconducting single-walled carbon nanotubes (SWCNTs) are molecular carbon cylinders that emit near-infrared (NIR-II, 1.1-1.4 µm) exciton PL.15 Owing to their remarkable photostability, NIR-II emission (which penetrates deeply in biological tissues with minimal autoscattering), and availability of a rich library of structures known as (n, m) chiralities, SWCNTs have attracted considerable research interest in developing innovative biomedical imaging capabilities.16–19 However, the chemical selectivity of unfunctionalized nanotubes is generally poor,20,21 which makes it difficult to differentiate specific chemicals such as H+ in the complex chemical environment of biological systems.

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Herein, we describe the first pH and temperature bi-functional optical nanoprobe based on a new sensing mechanism enabled by infrared defect PL of covalently functionalized semiconducting SWCNTs (f-SWCNTs). Covalently attaching an aminoaryl functional group to a SWCNT introduces an sp3 defect in the sp2 carbon lattice, creating an optically allowed defect state from which arises the defect PL (E11-), with a substantial red-shift from the original E11. Protonation of the amine substituent substantially modifies the energy level of the sp3 defect state, resulting in an even further red-shifted E11- emission. Therefore, this chemical event can be precisely reported as an energy shift in the defect PL, relative to a built-in reference, E11, whose position stays unchanged with pH (< 2 nm). When the nanotube is optically excited, the mobile excitons can become trapped efficiently at the proton-selective sp3 defect center and are recombined locally such that the nanotube acts as a light-harvesting antenna that amplifies the chemical information encoded by the defect PL (Figure 1). Furthermore, the solution temperature can be simultaneously measured from the E11/E11- intensity ratio because in the same chemical environment the relative population of E11 and E11- excitons is determined solely by the local temperature and closely follows the van’t Hoff equation. The proposed bi-functional nanoprobe is made possible by an unexpected finding from our team that controlled functionalization of SWCNTs with aryl functional groups creates a new, ultra-bright defect PL in the NIR-II.22 Defect PL is also observed in oxygen doped SWCNTs23 and with alkyl functional groups,24,25 although in the latter two cases, the PL is not as bright. Notably, the photoluminescence of f-SWCNTs responds sensitively to specific chemicals and biomolecules, in stark contrast with unfunctionalized SWCNTs. Our finding opens the possibility of covalent engineering of SWCNTs for optical biosensing applications. 2. EXPERIMENTAL SECTION 3



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2.1. Chirality-enriched SWCNTs. HiPco SWCNTs (Rice University, batch # 194.3) were dispersed in 1 wt.% sodium dodecyl sulfate (SDS) (Sigma Aldrich, ≥98.5) in deuterium oxide (D2O) (Cambridge Isotope Laboratories, Inc., 99.8%) by tip ultrasonication (Misonix) at 35 W (20% amplitude) and 10 oC in a stainless steel beaker for 2 hours, followed by ultracentrifugation with an Optima LE-80K Ultracentrifuge (Beckman Coulter) at 170,499g for 2 hours. The top three quarters of the supernatant was transferred and purified by gel chromatography26 using SephacrylTM S-200 high resolution chromatography resin (GE Healthcare) to give chiralityenriched SWCNT samples. All samples were diluted to 1 wt.% SDS in D2O with an optical density of 0.1 at the E11 absorption wavelength. The concentration of (6,5)-enriched solutions was calculated based on previous work by Zheng et al.27 2.2. Creation of sp3 defect sites on SWCNTs with diazonium chemistry. Aryl diazonium salts were freshly synthesized from anilines (Sigma Aldrich) following a modified literature method28 except 4-diazo-N,N-diethylaniline tetrafluoroborate, which was used as received from MP Biomedicals. The diazonium salts were dissolved and diluted in 1 wt.% SDS/D2O to give 10-2 to 10-6 M solutions, which were then reacted with the SWCNTs solutions. The reaction mixture was stirred at room temperature and protected from light by covering the reaction vial with aluminum foil. Reaction evolution was monitored in situ by UV-Vis-NIR absorption spectroscopy using a Lambda 1050 UV-Vis-NIR spectrophotometer (Perkin Elmer) and by photoluminescence spectroscopy with a NanoLog spectrofluorometer (Horiba Jobin Yvon). The spectrofluorometer was equipped with a 450 W Xenon arc lamp and a liquid-N2 cooled InGaAs array detector. The slit widths for excitation and emission were 10 nm and 20 nm, respectively. The reactions completed after 10 days or when stable E11- photoluminescence was established.

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2.3. pH-dependent defect photoluminescence. The solution pH was adjusted by adding small aliquots of 0.02 M HCl and NaOH solutions (Sigma Aldrich) and was determined by pH meter (AccumetTM AB15+ Basic and BioBasic pH meters and Accumet TM pH/ATC electrodes, Fisher Scientific). The pH-sensitive PL was obtained when the integrated area of E11- was greater than that of E11. 2.4. Temperature-dependent defect photoluminescence. The functionalized-SWCNT solution was heated from 15 oC to 85 oC using a circulating water bath, and the photoluminescence was obtained in increments of 5 oC. The solution temperature was measured with the Surface Temperature Sensor and LabQuest 2 (Vernier). The PL peaks of E11 and E11- were fitted with Voigt line shapes using Peakfit v4.12 (SeaSolve), then the integrated peak areas were plotted as a function of temperature. To avoid peak fitting artifacts, we compared different peak fitting parameters and confirm the robustness of fitting which resulted in the same relative potential well depth between the fully protonated (pH 3.77) and the fully de-pronated states (pH 9.01). 2.5. Defect photoluminescence as a pH sensor in biological medium. For the biological medium test, 0.3 mL of N,N-diethylaminoaryl f-SWCNTs in 1 wt.% SDS-D2O (optical density ~0.1 at E11, [C, CNT Carbon]/[Dz, Diazonium] = 1:100) was added to 1.2 mL of 10% v/v fetal bovine serum (Sigma-Aldrich) and DMEM media (Invitrogen) with 150 mM sodium chloride salt (EMP chemicals, ACS grade). To determine the influence of salts, 0.2 mL of f-SWCNTs in 1 wt.% SDS-D2O was added to 1.3 mL sodium chloride solutions of 50, 100, 150, and 300 mM. The emission spectra of the samples were taken at 565 nm excitation with adjusted integration time.

3. RESULTS AND DISCUSSION

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N,N-diethyl-4-aminobenzene functional groups were covalently attached to the sidewalls of (6,5)-SWCNTs via diazonium chemistry to create discrete, pH and temperature-sensitive defect centers (Figure 1). The surface density of the functional groups is kept low, approximately one every 20-100 nm, in order to maintain the E11 PL as an inherent reference while introducing the defect PL (E11-) as a probe. The functional density was confirmed by correlated PL, UV-VisNIR absorption, Raman scattering and X-ray Photoelectron Spectroscopy.22 The functionalized nanotubes, (6,5)-SWCNT-C6H4N(CH2CH3)2, showed two distinct, near-infrared emission peaks at pH 7.40, including E11 emission at 979 nm as well as E11- emission at 1120 nm (Figure S1). The E11 peak position is independent of pH and therefore can be used as an internal reference (Figure 2A). On the contrary, defect PL of (6,5)-SWCNT-C6H4N(CH2CH3)2 demonstrates a strong pH-dependence that closely traces the reproducible titration curve of the aminobenzene group (Figure 2B and 2C). Within the pH window from 4.5 to 8.5, E11- responds to pH change by shifting the PL peak position, creating a working window spanning 4 pH units. In particular, this working window completely covers the physiologic pH range for biological fluids (pH 5.5 to 8.0). The defect PL is resolvable to changes of 0.2 pH units within pH 5.5 to 8.0 and 0.3 pH units across pH 4.0 to 8.0. This sensitivity is among the highest reported for nanoprobes.5 Above pH 8.5 (or below pH 4.5), the emission vs pH curve plateaus as expected for a fully de-protonated (or protonated) state. Taking the first-order derivative (Figure 2C), the pKa of the covalently attached aminobenzene group was determined to be 6.28. This value is in good agreement with the pKa of N,N-dimethylammonium benzene (pKa = 5.2); the slightly basic nature of the nanoprobe can be attributed to the influence of the electron-rich SWCNTs. The positive

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correlation strongly suggests that the observed pH dependence originates from the covalently attached aminoaryl functional groups on carbon nanotubes. The pH-dependent response of the defect PL is correlated with the equilibrium between the protonated and deprotonated forms of the amine substituents: [CNT-BH+] ⇄ [CNT-B] + [H+]

eq (1)

where [CNT-BH+] and [CNT-B] are the concentration of CNT-anchored C6H4N+(CH2CH3)2H, C6H4N(CH2CH3)2, respectively. From the Henderson-Hasselbalch equation, pH

pKa

log

eq (2)

and the fact that the concentration is directly proportional to the fluorescence intensity in the dilute limit, the pH is determined as pH where

,

and

,

6.28

C ∗ log

, ,

eq (3)

are the integrated area of defect PL from amine and ammonium

functional groups, respectively; and C is a correction factor, which is experimentally determined as 2.04±0.03 for (6,5)-SWCNT-C6H4N(CH2CH3)2 (Figure S2). The correction factor arises from the difference in quantum yields between the protonated and deprotonated states of the defect photoluminescence. This factor may be dependent on the nanotube chirality, as suggested by the chirality-dependent defect photoluminescence. Although we do not have the data points to quantitatively define “C” for each chirality other than (6,5)-SWCNTs, this may be an interesting question for follow-up experiments as pure samples become available for other chiralities. Control experiments with a series of para-substituted aryl functional groups confirm that only amine moieties show pronounced photoluminescence pH effects (Table 1). Identical pHdependences were observed in both N,N-dimethylamino- and N,N-diethylaminobenzene functionalized SWCNTs. In contrast, other terminating moieties such as methoxy, bromo, and 7



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nitro, did not show pH-responsive E11 or E11- emission. This observation can be related to the electronic resonance and inductive effects of a substituent on the benzene ring, which is referred to as the Hammett constant (σp). For the aryl group, the protonation of amine to ammonium transitions the Hammett constant from -0.66 to +0.82.29 This significant change in electronic withdrawing capability lowers the energy level of the lowest unoccupied molecular orbital (LUMO) resulting in a further redshifted defect emission peak. In addition, a linear relationship between the energy shift (E11 - E11-) and the Hammett constant is consistent with our previous work.22 In the case of SWCNT-C6H4COOH, no significant pH dependence was observed, which is understood by the much smaller Hammett constant difference of 0.43 between the protonated and de-protonated forms of carboxylic acid compared to ~1.50 in the case of the aminoaryl group. The pH-responsive defect PL features a strong dependence on the chiral structure of SWCNTs (Figure 3, Figure S3 and Table S1). The optical response to pH change (between pH 4.0 and pH 9.0) ranges from 18 to 33 meV for five different SWCNT chiralities with N,Ndiethylaminobenzene functional groups. In both acidic and basic conditions, the defect PL shift shows a 1/d2 dependence on the nanotube diameter d. In general, the smaller the diameter, the larger the pH response. Among the investigated species, the aminobenzene functionalized (8,3)SWCNT has the highest energy shift (33 meV). This diameter dependence is evidence for the dark exciton brightening mechanism, which leads to the enhanced, red-shifted E11photoluminescence.22 Independent of pH-responsive energy shifts, the intensity of the defect PL exhibits a strong dependence on temperature (Figure 4A). Within a broad temperature range including the biological environment from 25 to 45 oC, the intensities of E11- and E11 are ruled by the statistically determined Boltzmann factor. Due to the finite well depth of the defect state, a

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trapped exciton can escape with the aid of thermal energy, kBT, resulting in redistribution of the E11- and E11 exciton populations (Figure 4B). The PL intensity ratio of E11- and E11 correlates directly with temperature as described by the van’t Hoff equation: ln where

and

∆

C′

eq (4)

are the integrated PL intensities of the original and defect exciton emissions,

respectively; ∆E′ is the potential well depth of the defect state; kB is the Boltzmann constant; T is temperature; and C′ is a constant. By monitoring the photoluminescence of (6,5)-SWCNTC6H4N(CH2CH3)2 under constant pH while increasing temperature from 15 to 85 oC, we observed the temperature-responsive PL (Figure 4C). We note that a different temperature calibration is required for each pH because the acid-base equilibrium of the amino moiety is temperature dependent. Although the original exciton PL (E11) from pristine SWCNTs can be responsive to temperature,30 it is also susceptible to other environmental fluctuations. In contrast, the NIR E11-/E11 intensity ratio provides a reliable measurement of local temperature because both the probe (E11-) and internal reference (E11) are maintained in the same chemical environment. We note that the temperature dependence of defect photoluminescence has been previously reported by Weisman and coworkers.23 However, to the best of our knowledge, this is the first report of an optical nanothermometer based on this phenomenon. Furthermore, from the evolving intensity ratio of E11/E11- over temperature, we have constructed the van’t Hoff plots (Figure 4C) to extract the potential well depth (∆E') of the defect state, as described previously by Ghosh et al.23 This well depth is directly related to the energy difference between the LUMOs of E11 and E11-. From the van’t Hoff plots of aminoaryl (6,5)SWCNTs, we derived a well depth of 103 meV for the de-protonated form and 130 meV for the protonated form. The difference, 27 meV, is 9 meV larger than the corresponding PL emission 9



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shift. This difference may be attributed to an asymmetric effect of protonation on the energy levels of LUMO and HOMO, as illustrated in Figure 4D. Importantly, the ammonium groups (HR2N+), with a Hammett constant of ~ +0.82, have a deep potential well of 130 meV, which is comparable to our experiments with nitro groups (σp ~0.778, ∆E

138 meV). This result

confirms that the pH dependence arises from energy modulation of the defect state through changes in the local environment of covalently attached aminoaryl groups. To illustrate the prospect of the observed phenomenon as optical pH sensors, we measured the pH of 10% v/v fetal bovine serum in Dulbecco's Modified Eagle's medium using SDS-stablized N,N-diethylaminobenzene-(6,5)-SWCNTs (Figure S4). We found that the optically measured pH values are in good agreement with readings from a pH meter. Previous works by Dai et al., Strano et al. and others suggest that SDS can be replaced with a bio-friendly molecule such as phospholipid-polyethylene glycol (PL-PEG) and Pluronic F68 (PF68).19,31–33 Due to the low density of surface functional groups in our samples, we expect that these previous findings can be applied directly to our system to create a biocompatible sensor based on pHdependent defect PL. Ongoing experiments will verify this hypothesis. Conveniently, this defect PL probe can be excited by both visible (though E22 excitation) or the near IR (through E11 excitation), as we have shown previously,24 to yield E11- PL in NIR-II that reduces autofluorescence and photo damage. Importantly, our bi-functional nanosensor measures pH by the energy difference between the probe (E11-) and an internal reference (E11). This sensing mechanism is distinctly different from all previous nanotube-based optical sensors, which is based on photoluminescence quenching of E11. From a previous work by Strano et al.,34 the E11 absorption and photoluminescence becomes diminishing at low pH. We note that in the case of aminoaryl-

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SWCNTs, the PL intensity dropped by 33% when the pH decreased from 9.02 to 4.93, as shown in Figure S5. However, the PL intensity is adequately strong for the proposed sensing applications and can even more stable over a wide pH range by replacing SDS with other surfactants as suggested by Duque et al.35 The photoluminescence intensity of E11 can also be influenced by the surrounding environment such as salts and surfactants.36–40 Salts can modify surfactant wrapping resulting in change in E11 PL intensity, as previously reported by Ziegler et al.37 and Doorn et al.38 The PL intensity of E11 tends to decrease at high salt concentrations (> 450 mM).38 However, this salt influence becomes intrinsically irrelevant here, again, because the pH is measured from the energy difference between E11 and E11-, not the intensity. The photoluminescence intensity becomes a concern only when the signal is too weak. For aminoaryl-(6,5), only when the salt concentration exceeds 300 mM, the drop in PL intensity becomes more substantial (by 55% at pH 5.5 and ~67% at pH 9.0). However, even under the salt level of serum (135 - 145 mEq/L), the pH-dependent energy shift from the fluorescent nanoprobe agrees well with the pH meter (Figure S6).

4. CONCLUSIONS We show that protonation of covalently attached aminoaryl groups on semiconducting carbon nanotubes can be probed by pH-dependent defect photoluminescence. Switching between the protonated and de-protonated states of the amino moiety significantly modifies the defect state of the covalently functionalized SWCNTs, producing energy shifts as large as 33 meV. This novel defect sensor responds sensitively and selectively to pH changes as small as 0.2 pH units over a wide working window (pH 4.5 to 8.5) that fully covers the pH range of typical biological fluids. Compared to ultra-pH-sensitive nanoparticles,5 which are based on pH-responsive self11



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assembly of fluorescent and quenching building blocks, our nanoprobe does not require a library of nanoparticles to cover the biologically relevant range of pH, which may be advantageous for high resolution imaging. Our experiments further uncover a strong temperature dependence of the defect photoluminescence that correlates the relative E11-/E11 population with temperature by the van’t Hoff equation as a tool for optical probing. Based on these findings, the first bifunctional optical nanoprobe is proposed to offer simultaneous pH and temperature sensing capabilities with a built-in reference. Featuring a unique combination of high photostablility, high sensitivity, high selectivity, wide working window, nanoscale size, and inherently low background of NIR-II photoluminescence, this novel nanoprobe may find applications in chemical imaging, biosensing and chemo-photothermal therapy at the cellular and even molecular levels.

ASSOCIATED CONTENT Supporting Information Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Tel.: +1-301-405-3368. Fax: +1-301-314-9121. E-mail: [email protected]. Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the University of Maryland startup funds and the National Science Foundation (CAREER CHE-1055514). We thank Philip G. Collins for constructive suggestions.

REFERENCES (1) Engel, T.; Reid, P. Physical Chemistry 3rd edition, Prentice Hall, U.S., 2012. (2) Lagadic-Gossmann, D.; Huc, L.; Lecureur, V. Alterations of Intracellular pH Homeostasis in Apoptosis: Origins and Roles. Cell Death Differ. 2004, 11, 953–961. (3) Webb, B. A.; Chimenti, M.; Jacobson, M. P.; Barber, D. L. Dysregulated pH: A Perfect Storm for Cancer Progression. Nat. Rev. Cancer 2011, 11, 671–677. (4) Wijesinghe, D.; Arachchige, M. C. M.; Lu, A.; Reshetnyak, Y. K.; Andreev, O. A. pH Dependent Transfer of Nano-Pores into Membrane of Cancer Cells to Induce Apoptosis. Sci. Rep. 2013, 3. (5) Ma, X.; Wang, Y.; Zhao, T.; Li, Y.; Su, L.-C.; Wang, Z.; Huang, G.; Sumer, B. D.; Gao, J. Ultra-pH-Sensitive Nanoprobe Library with Broad pH Tunability and Fluorescence Emissions. J. Am. Chem. Soc. 2014. (6) Wencel, D.; Abel, T.; McDonagh, C. Optical Chemical pH Sensors. Anal. Chem. 2014, 86, 15–29. (7) Brites, C. D. S.; Lima, P. P.; Silva, N. J. O.; Millán, A.; Amaral, V. S.; Palacio, F.; Carlos, L. D. Thermometry at the Nanoscale. Nanoscale 2012, 4, 4799–4829. (8) Ozawa, T.; Yoshimura, H.; Kim, S. B. Advances in Fluorescence and Bioluminescence Imaging. Anal. Chem. 2013, 85, 590–609. (9) Lee, J.; Kotov, N. A. Thermometer Design at the Nanoscale. Nano Today 2007, 2, 48–51. (10) Tang, R.; Lee, H.; Achilefu, S. Induction of pH Sensitivity on the Fluorescence Lifetime of Quantum Dots by NIR Fluorescent Dyes. J. Am. Chem. Soc. 2012, 134, 4545–4548. (11) Aigner, D.; Borisov, S. M.; Petritsch, P.; Klimant, I. Novel near Infra-Red Fluorescent pH Sensors Based on 1-Aminoperylene Bisimides Covalently Grafted onto Poly(acryloylmorpholine). Chem. Commun. 2013, 49, 2139–2141. (12) Yang, J.-M.; Yang, H.; Lin, L. Quantum Dot Nano Thermometers Reveal Heterogeneous Local Thermogenesis in Living Cells. ACS Nano 2011, 5, 5067–5071. (13) Takei, Y.; Arai, S.; Murata, A.; Takabayashi, M.; Oyama, K.; Ishiwata, S.; Takeoka, S.; Suzuki, M. A Nanoparticle-Based Ratiometric and Self-Calibrated Fluorescent Thermometer for Single Living Cells. ACS Nano 2014, 8, 198–206. (14) He, C.; Lu, K.; Lin, W. Nanoscale Metal–Organic Frameworks for Real-Time Intracellular pH Sensing in Live Cells. J. Am. Chem. Soc. 2014, 136, 12253–12256. 13



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(15) O’Connell, M. J. Band Gap Fluorescence from Individual Single-Walled Carbon Nanotubes. Science 2002, 297, 593–596. (16) Cognet, L.; Tsyboulski, D. A.; Rocha, J.-D. R.; Doyle, C. D.; Tour, J. M.; Weisman, R. B. Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single-Molecule Reactions. Science 2007, 316, 1465–1468. (17) Heller, D. A.; Jeng, E. S.; Yeung, T.-K.; Martinez, B. M.; Moll, A. E.; Gastala, J. B.; Strano, M. S. Optical Detection of DNA Conformational Polymorphism on Single-Walled Carbon Nanotubes. Science 2006, 311, 508–511. (18) Smith, A. M.; Mancini, M. C.; Nie, S. Bioimaging: Second Window for in Vivo Imaging. Nat. Nanotechnol. 2009, 4, 710–711. (19) Welsher, K.; Liu, Z.; Sherlock, S. P.; Robinson, J. T.; Chen, Z.; Daranciang, D.; Dai, H. A Route to Brightly Fluorescent Carbon Nanotubes for near-Infrared Imaging in Mice. Nat. Nanotechnol. 2009, 4, 773–780. (20) Huang, J.; Ng, A. L.; Piao, Y.; Chen, C.-F.; Green, A. A.; Sun, C.-F.; Hersam, M. C.; Lee, C. S.; Wang, Y. Covalently Functionalized Double-Walled Carbon Nanotubes Combine High Sensitivity and Selectivity in the Electrical Detection of Small Molecules. J. Am. Chem. Soc. 2013, 135, 2306–2312. (21) Kam, N. W. S.; O’Connell, M.; Wisdom, J. A.; Dai, H. Carbon Nanotubes as Multifunctional Biological Transporters and near-Infrared Agents for Selective Cancer Cell Destruction. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 11600–11605. (22) Piao, Y.; Meany, B.; Powell, L. R.; Valley, N.; Kwon, H.; Schatz, G. C.; Wang, Y. Brightening of Carbon Nanotube Photoluminescence through the Incorporation of sp3 Defects. Nat. Chem. 2013, advance online publication. (23) Ghosh, S.; Bachilo, S. M.; Simonette, R. A.; Beckingham, K. M.; Weisman, R. B. Oxygen Doping Modifies Near-Infrared Band Gaps in Fluorescent Single-Walled Carbon Nanotubes. Science 2010, 330, 1656–1659. (24) Zhang, Y.; Valley, N.; Brozena, A. H.; Piao, Y.; Song, X.; Schatz, G. C.; Wang, Y. Propagative Sidewall Alkylcarboxylation That Induces Red-Shifted Near-IR Photoluminescence in Single-Walled Carbon Nanotubes. J. Phys. Chem. Lett. 2013, 4, 826–830. (25) Brozena, A. H.; Leeds, J. D.; Zhang, Y.; Fourkas, J. T.; Wang, Y. Controlled Defects in Semiconducting Carbon Nanotubes Promote Efficient Generation and Luminescence of Trions. ACS Nano 2014, 8, 4239–4247. (26) Liu, H.; Nishide, D.; Tanaka, T.; Kataura, H. Large-Scale Single-Chirality Separation of Single-Wall Carbon Nanotubes by Simple Gel Chromatography. Nat. Commun. 2011, 2, 309. (27) Zheng, M.; Diner, B. A. Solution Redox Chemistry of Carbon Nanotubes. J. Am. Chem. Soc. 2004, 126, 15490–15494. (28) Quintero, B.; Cabeza, M. C.; Martínez, M. I.; Gutiérrez, P.; Martínez, P. J. Dediazoniation of P-Hydroxy and P-Nitrobenzenediazonium Ions in an Aqueous Medium: Interference by the Chelating Agent Diethylenetriaminepentaacetic Acid. Can. J. Chem. 2003, 81, 832–839. (29) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165–195. (30) Berger, S.; Voisin, C.; Cassabois, G.; Delalande, C.; Roussignol, P.; Marie, X. Temperature Dependence of Exciton Recombination in Semiconducting Single-Wall Carbon Nanotubes. Nano Lett. 2007, 7, 398–402.

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Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Diao, S.; Hong, G.; Robinson, J. T.; Jiao, L.; Antaris, A. L.; Wu, J. Z.; Choi, C. L.; Dai, H. Chirality Enriched (12,1) and (11,3) Single-Walled Carbon Nanotubes for Biological Imaging. J. Am. Chem. Soc. 2012, 134, 16971–16974. (32) Singh, R. P.; Jain, S.; Ramarao, P. Surfactant-Assisted Dispersion of Carbon Nanotubes: Mechanism of Stabilization and Biocompatibility of the Surfactant. J. Nanoparticle Res. 2013, 15, 1–19. (33) Iverson, N. M.; Barone, P. W.; Shandell, M.; Trudel, L. J.; Sen, S.; Sen, F.; Ivanov, V.; Atolia, E.; Farias, E.; McNicholas, T. P.; et al. In Vivo Biosensing via Tissue-Localizable nearInfrared-Fluorescent Single-Walled Carbon Nanotubes. Nat. Nanotechnol. 2013, 8, 873–880. (34) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; et al. Reversible, Band-Gap-Selective Protonation of Single-Walled Carbon Nanotubes in Solution. J. Phys. Chem. B 2003, 107, 6979– 6985. (35) Duque, J. G.; Cognet, L.; Parra-Vasquez, A. N. G.; Nicholas, N.; Schmidt, H. K.; Pasquali, M. Stable Luminescence from Individual Carbon Nanotubes in Acidic, Basic, and Biological Environments. J. Am. Chem. Soc. 2008, 130, 2626–2633. (36) Miyauchi, Y.; Saito, R.; Sato, K.; Ohno, Y.; Iwasaki, S.; Mizutani, T.; Jiang, J.; Maruyama, S. Dependence of Exciton Transition Energy of Single-Walled Carbon Nanotubes on Surrounding Dielectric Materials. Chem. Phys. Lett. 2007, 442, 394–399. (37) Silvera-Batista, C. A.; Scott, D. C.; McLeod, S. M.; Ziegler, K. J. A Mechanistic Study of the Selective Retention of SDS-Suspended Single-Wall Carbon Nanotubes on Agarose Gels. J. Phys. Chem. C 2011, 115, 9361–9369. (38) Niyogi, S.; Boukhalfa, S.; Chikkannanavar, S. B.; McDonald, T. J.; Heben, M. J.; Doorn, S. K. Selective Aggregation of Single-Walled Carbon Nanotubes via Salt Addition. J. Am. Chem. Soc. 2007, 129, 1898–1899. (39) Niyogi, S.; Densmore, C. G.; Doorn, S. K. Electrolyte Tuning of Surfactant Interfacial Behavior for Enhanced Density-Based Separations of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 1144–1153. (40) Duque, J. G.; Densmore, C. G.; Doorn, S. K. Saturation of Surfactant Structure at the Single-Walled Carbon Nanotube Surface. J. Am. Chem. Soc. 2010, 132, 16165–16175.

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1 Schematiic illustratio on of NIR-III pH sensoor based onn carbon naanotube anteennaFigure 1. amplified d defect photoluminesscence. Pro otonation off an aminoobenzene ffunctional ggroup covalentlly attached on a (6,5)-S SWCNT mo odifies the ennergy level of the sp3 ddefect state from which a new photolu uminescencee arises. Thee nanotube aacts as an opptical antennna that efficiiently he generated d excitons tto the defect site, where the exccitons harvests light and channels th recombin ne to produce near-infrarred photolum minescence eencoding thee chemical innformation aat the functionaal groups (reed arrow).

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C

B

A

1140

1.0

*

0.5 E11

E11

-

E11

18 meV

990

1130

1120 E11

980

0.0

d(E11-)/d(pH)

1.5

0

1000

E11- Emission (nm)

pH 3.97 pH 9.02

E11 Emission (nm)

Relative PL Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


biological pH window

-1 -2 -3 -4

6.28

1110 1000

1100

Emission (nm)

1200

3

4

5

6

7

8

9

pH

3

4

5

6

7

8

9

pH

Figure 2. Defect photoluminescence of (6,5)-SWCNT-C6H4N(CH2CH3)2 is strongly dependent on pH. A) The E11- emission of the N,N-diethyl-4-aminobenzene functionalized (6,5)-SWCNTs redshifts from 1117 nm to 1136 nm, as the pH changes from 9.02 to 3.97. The E11 and E11emission curves (solid lines) are fitted and plotted on top of the original data (dotted lines). The difference is due to (6,4)-SWCNT-C6H4N(CH2CH3)2 impurity in the solution, marked with an asterisk (*). B) The E11- emission responds sensitively to the pH change whereas E11 remains unchanged. The maximum energy difference between the protonated and the de-protonated state is ~18 meV in the case of (6,5)-SWCNT-C6H4N(CH2CH3)2. C) The first derivative of the relative PL energy shift of E11 and E11- as a function of pH determines the pKa for the nanoprobe.

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E11- (nm)

(6,5)-ArX

E11 (nm)

pH 9.0

pH 4.0

E11- shift (meV)

Pristine

971

-

-

-

-N(CH2CH3)2

975

1116

1134

18

-N(CH3)2

975

1116

1134

18

-OCH3

973

1116

1116

0

-COOH

976

1124

1124

0

-Br

973

1125

1125

0

-NO2

973

1137

1137

0

-3,5-NO2

973

1145

1145

0

Table 1. Defect photoluminescence of (6,5)-SWCNTs with various functional groups –ArX at different pH. The pH dependence is observed only in the aminobenzene functionalized (6,5)SWCNTs.

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B pH 9.0 pH 4.0

700

(7,6)

(6,5) (8,4) (6,4) (8,3)

500

2.2 2.0 1.8 1.6 1.4 1.2 1/d2 (nm-2)

E11

-

(8,3) -

pH 9.0

700 600

E11 E11 (8,4)

600 500

600

pH 9.0

x4

700

0 600

500

-0.3

E11

700

(6,5) (6,4) (6,5) E11 E11- E11-

600 Excitation (nm)

-0.1

-0.2

C

Excitation (nm)

A 0.0

E11- - E11 (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pH 4.0 900

1000

1100

1200

Emission (nm)

500

pH 4.0

x4 1000

1200

1400

Emission (nm)

Figure 3. Chirality dependent pH response of N,N-diethylaminobenzene functionalized SWCNTs. A) The energy shifts between pH 9.0 and pH 4.0 (ΔE) vary from 18 to 33 meV depending on the nanotube chirality. Defect photoluminescence of B) (6,5)/(6,4)-SWCNTC6H4N(CH2CH3)2 and C) (8,3)/(8,4)-SWCNT-C6H4N(CH2CH3)2.

19



A

B kBT

2.0

CB E'

E11

1.5 1.0 20 40 60 80

o (C )

0.5 0.0

E11-

E11

1000 1200 1400 Emission (nm)

Te mp era tur e

Relative PL Intensity

E11-

C

VB

Defect

D

CB

0.0

0.103 eV

pH 3.77 pH 9.01

0.130 eV

LUMO

-0.5 ln (IE11/IE11-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

E11 Pristine 1.27 eV

-1.0

-

E11 pH 9.01 1.11 eV

-

E11 pH 3.77 1.09 eV

-1.5 HOMO

0.0028 0.0030 0.0032 0.0034 1/T (K-1)

VB

Figure 4. Temperature-dependent defect photoluminescence of (6,5)-SWCNT-C6H4N(CH2CH3)2. A) Evolution of E11 and E11- PL from protonated (6,5)-SWCNT-C6H4N(CH2CH3)2 with increasing temperature at pH 3.77. B) Schematic illustration of thermal promotion of an exciton from the defect state (E11-) to the higher E11 state. The relative exciton population is correlated with the relative energy of the defect states. C) The van’t Hoff plots (the E11/E11- integrated intensity ratios as a function of temperature) at pH 3.77 (red) and pH 9.01 (blue). The slope of linear regression is used to determine the relative E11- energy between the protonated and de20


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protonateed states off the aminob benzene sub bstituents. D D) Simplifieed energy ddiagram of ((6,5)SWCNT-C6H4N(CH H2CH3)2 at pH H 3.77 and pH 9.01.

Table off Contents Im mage

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