Highly Selective Detection of Carbon Monoxide in Living Cells by

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Highly Selective Detection of Carbon Monoxide in Living Cells by Palladacycle Carbonylation-Based Surface Enhanced Raman Spectroscopy Nanosensors Yue Cao,† Da-Wei Li,*,† Li-Jun Zhao, Xiao-Yuan Liu, Xiao-Ming Cao, and Yi-Tao Long* Key Laboratory for Advanced Materials & Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China

Downloaded by TEXAS A&M INTL UNIV on September 11, 2015 | http://pubs.acs.org Publication Date (Web): September 8, 2015 | doi: 10.1021/acs.analchem.5b01793

S Supporting Information *

ABSTRACT: A novel nanosensor was explored for the highly selective detection of intracellular carbon monoxide (CO) by surface enhanced Raman spectroscopy (SERS) on the basis of palladacycle carbonylation. By assembling new synthesized palladacycles (PC) on the surface of gold nanoparticles (AuNPs), SERS nanosensors (AuNP/PC) were prepared with good SERS activity and reactivity with CO. When the AuNP/PC nanosensors were incubated with a CO-containing system, carbonylation of the PC assembled on AuNPs was initiated, and the corresponding SERS spectra of AuNP/PC changed significantly, which allowed the carbonylation reaction to be directly observed in situ. Upon SERS observation of COdependent carbonylation, this SERS nanosensor was used for the detection of CO under physiological conditions. Moreover, benefiting from the specificity of the reaction coupled with the fingerprinting feature of SERS, the developed nanosensor demonstrated high selectivity over other biologically relevant species. In vivo studies further indicated that CO in normal human liver cells and HeLa cells at concentrations as low as 0.5 μM were successfully detected with the proposed SERS strategy, demonstrating its great promise for the analytical requirements in studies of physiopathological events involved with CO.

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of high selectivity, high sensitivity, rapid response, and nondestructive detection.18 SERS also displays significant advantages over fluorescence in terms of its resistance to photobleaching and phototoxicity.19 These comprehensive advantages render SERS highly competitive to meet the needs of the detection of biological species in living cells. For instance, the in vivo detection of tumor biological markers has been achieved by SERS with high selectivity.11 These achievements reveal exciting prospects of SERS for analyzing bioorganic molecules; however, direct SERS detection remains challenging for small inorganic species because of their small Raman scattering cross sections. This challenge could be overcome by using SERS-active metal nanoparticles integrated with Raman reporters, which have high SERS responsiveness and good recognition capability for inorganic species.20 In this way, based on the variation of the SERS spectra after recognizing a reaction, a few inorganic species, such as metal ions and superoxide radicals, have be detected.21,22 However, to the best of our knowledge, the detection of intracellular CO with SERS has not been reported.

arbon monoxide (CO) has been increasingly recognized as an important gaseous messenger in mammalian cells, although it was previously viewed as a toxicant or pollutant.1−3 Emerging studies demonstrate that, akin to nitric oxide (NO) and hydrogen sulfide (H2S), CO plays a significant role in signaling pathways involved in a number of pathophysiological processes, including neurotransmission,4,5 vasodilation,6 antiinflammatory response,7,8 and antiproliferative activities.9 Consequently, CO has also drawn attention as a promising therapeutic reagent for various human diseases.10,11 However, although CO is proved to have diverse effects in biological processes, many aspects of its functions are still indistinct.12 To elucidate the biological roles of CO, several techniques, such as infrared absorption, chromatography,13 chromogenic detection,14 and electrochemical assays,15 have been developed. These methods allow for CO analysis, but they are difficult to implement for the detection of CO in biological circumstances in a noninvasive way.15 Fluorescence sensing may be a more powerful technique to detect CO in living systems.11,12 However, the fluorescence analysis for living cells probably suffers from the photobleaching and phototoxicity.16 Surface-enhanced Raman spectroscopy (SERS) has emerged as a promising analytical technique in virtue of magnifying the vibrational fingerprints of analytes with plasmonic structures.17 SERS has a distinguishing feature of combining the advantages © XXXX American Chemical Society

Received: May 13, 2015 Accepted: September 1, 2015

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DOI: 10.1021/acs.analchem.5b01793 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

% HAuCl4 under continuous stirring. When the color of the solution changed from deep-blue to wine-red within 3 min, the reaction flask was cooled to room temperature and the stirrer was turned off. To fabricate AuNP/PC, the prepared AuNPs were incubated with a certain concentration of PC solution for 5 min. Then, the excess PC and other possible impurities were washed away by centrifugation and subsequent resuspension with deionized water (three times). Characterization of AuNP/PC Nanosensors. Transmission electron microscopic (TEM) images were obtained on a JEM-2100 transmission electron microscope (JEOL, Tokyo, Japan) equipped for analysis at an accelerating voltage of 200 kV. The Raman spectra of the relative samples were measured with an inverted microscope with an external triple channel optical system (Ti−U, Nikon, Japan) equipped with a 40× plane objective. A 100 W halogen tungsten lamp equipped with dark-field condenser lens (0.8 < NA < 0.95, Nikon, Japan) was used to illuminate the nanoparticles. A 785 nm laser was adopted for Raman excitation in the front of a filter turret (TIFLC Epi-FL, Nikon, Japan). A back-illuminated CCD was mounted on the spectrometer with a 2 cm−1 resolution (ProEM 1600, Princeton Instruments, MA), allowing the fast and sensitive Raman measurement from 400 to 1800 cm−1. SERS Detection of CO in Solution. Solutions of CO releasing molecules (CORMs, tricarbonylchloro(glycinato)ruthenium(II)) at different concentrations were freshly prepared, and 50 μL of CORMs solution was immediately added to the prepared colloidal AuNP/PC. Then, in vitro SERS detection was conducted after a 30 min reaction performed in a water bath set to 37 °C. The selectivity of AuNP/PC nanosensors was tested by the SERS responses of possible interferences in the presence or absence of CO. Solutions of possible interferences were prepared as follows:11 5 μL of 10 mM stock solution of H2O2, tert-butyl hydroperoxide (TBHP), NaClO, KO2, Na2S, KCN, and GSH was added to 995 μL of 2 nM solution of AuNP/PC in phosphate buffer (PBS, pH = 7.4), respectively; ROO• radicals were prepared by thermal decomposition of 5.0 mM 2,2′-azobis[2-methylpropionamidine] dihydrochloride (AAPH) at 37 °C; 1.0 mM S-nitroso-Nacetyl-dl-penicillamine (SNAP) was dissolved into 100 mM PBS to produce nitric oxide (NO) at ambient temperature.22 SERS Detection of CO in Living Cells. HeLa and L02 cells were cultured in a flask in Dulbecco’s modified Eagle’s medium supplemented with heat-inactivated bovine serum (10%), 100 U/mL penicillin, and 100 U/mL streptomycin and were maintained at 37 °C in a humidified atmosphere (5% CO2 and 95% air). The cells were grown in dishes (6 cm in diameter) with 6 × 105 cells/dish and were allowed to adhere for 12 h. The cells incubated with a culture medium containing 2 nM AuNP/PC instead of the growth medium for 4 h and with CORMs at different concentration (0.5 μM, 5 μM, and 30 μM, respectively) for the final 45 min. Then the cells were subjected to washing with PBS. Subsequently, the culture dish was fitted to a small incubator on the microscope stage to conduct imaging and SERS measurement under cell culture conditions with 1 mW laser illumination and 5 s integration time.

To make good use of the SERS advantages for the detection of intracellular CO, a functional reagent is required, which could be conjugated on SERS-active nanoparticles and react with CO specifically. Palladacycle is a favorable candidate to meet this requirement because it can react with CO to generate carbonyl products with possible SERS changes.11,23,24 Therefore, in this work, a SERS approach was explored for CO detection based on the carbonylation of palladacycle with CO as the carbon (C1) source. As outlined in Scheme 1A,


Scheme 1. Schematic Illustration of SERS Nanosensors for the Detection of Intracellular COa

a

(A) SERS response and sensing mechanism of palladacycle carbonylation on AuNP/PC nanosensors for CO. (B) SERS detection of CO in living cells using AuNP/PC nanosensors.

palladacycles with amino groups (PC) were designed and synthesized, and then they were assembled on the surface of gold nanoparticles (AuNPs) with amine-gold links to construct a type of SERS nanosensor (AuNP/PC). Once CO bonds to the nanosensors, palladacycle carbonylation takes place and SERS spectrum of the nanosensors will change correspondingly. With the acquired new peak in the SERS spectra, the observation of the carbonylation reaction could be accomplished. On the basis of this, the AuNP/PC nanosensors can be applied to the selective and sensitive detection of CO in living cells (Scheme 1B), providing the great potential for the analysis of intracellular CO.

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EXPERIMENTAL SECTION Reagents. All reagents were of analytical grade and used without further purification. Potassium hexacyanoferrate (III) and hydrogen peroxide (H2O2, 30 wt %) were purchased from Aladdin Chemical Company (Shanghai, China). Gold(III) chloride hydrate (HAuCl4·3H2O), glutathione (GSH), and sodium hydrosulfide monohydrate (NaHS·H2O, ≥ 90%) were obtained from Sigma-Aldrich (St. Louis, MO). The normal human liver cell line (L02) and Hela cell line were purchased from the Chinese Academy of Sciences in Shanghai originally from American Type Culture Collection (Manassas, VA). Ultrapure water (18.2 MΩ cm) was used throughout the work. All reactions using air- or moisture-sensitive reagents were performed in dried glassware under an atmosphere of dry N2. Fabrication of AuNP/PC Nanosensors. AuNPs were prepared according to the process reported previously.25 Briefly, 5 mL of 1.0 wt % sodium citrate was added gradually into 100 mL of boiling water containing approximately 0.03 wt

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RESULTS AND DISCUSSION Fabrication and Characterization of AuNP/PC Nanosensors. PC was synthesized by the reduction of 4-nitro-N,Ndimethylbenzylamine to 4-amino-N,N-dimethylbenzylamine26 and subsequent cyclometalation with Li2PdCl4, which was B


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Figure 1. SERS observation of palladacycle carbonylation involved with CO. (A) SERS spectra collected from colloidal AuNP/PC incubated with saturated CO solution at different reaction times. (B) Typical TEM images from the colloidal AuNP/PC before and after reaction with CO. (C) XPS spectra of Pd 3d and Cl 2p regions for AuNP/PC before and after reaction with CO.

finally transformed to the chloride dimer (Supplementary Note 1−2, Figure S1−S5). Then, condensed AuNPs prepared according to the reported citrate reduction method25 were incubated with the PC solution to form a self-assembled monolayer (SAM) of palladacycle on the surface of AuNPs to construct a novel type of SERS nanosensors. The surface morphology of the obtained AuNP/PC nanosensors was characterized by scanning electron microscopy (SEM), and the typical SEM image indicated that the diameter of spherical AuNP/PC was approximately 60 nm (Figure S6). This implies good SERS activity and suitability for biological analysis because AuNPs with this size are efficient at enhancing Raman signals at near-infrared excitations, such as 785 nm.27,28 In further SERS tests (Figure S7A), the Raman responses of the AuNPs and PC solution were barely detected, whereas obvious Raman peaks could be observed for AuNP/PC. Because SERS is a surface selective technique, these Raman signals indicate that PC are successfully assembled on the surface of AuNPs through the amine-gold link and that the developed AuNP/PC are SRES active as expected without damping the plasmonic activity of AuNPs by the PC assembly. In addition, the SERS activity of AuNP/PC was maintained stable for more than 2 weeks (Figure S7B), which was further experimentally proved by no shift of the Raman peak with prolonged storage, implying few decompositions or other changes of AuNP/PC. Taken together, by assembling PC on the surface of AuNPs, SERS active nanostructures with good stability were successfully fabricated, which are beneficial for detection applications. SERS Observation of Palladacycle Carbonylation on AuNP/PC Nanosensors. The feasibility of SERS observation of palladacycle carbonylation was tested with the prepared AuNP/PC nanosensors. To perform the carbonylation reaction, AuNP/PC were redispersed in a cuvette with inlet and outlet ports using deionized water saturated with CO by continuous introduction of 5% CO/N2 gas (60 mL/min). The observation of the carbonylation process was conducted by

collecting the SERS spectrum from the AuNP/PC at different reaction times until there were no noticeable changes between adjacent spectra. According to the experimentally recorded spectra (Supplementary Note 3, Figure 1A, Table S1), we could obtain a preliminary understanding of the reaction process. Initially, the SERS spectrum of PC as a SAM on the surface of AuNPs was observed, among which the Raman band at 1173 cm−1 is caused by the symmetric stretching vibration of C−C, whereas the peaks at 1209 cm−1 are due to phenyl ring modes.29 After 10 min of reaction, the intensity of the PC bands at 1173 and 1209 cm−1 decreased significantly, whereas new vibrational Raman bands at 1032, 1118, and 1217 cm−1 appeared. These bands may result from the coordination of CO to palladium trans to the donor group and insertion in the Pd−C bond.23 Among the newly appeared bands, the very strong bands at 1032 cm−1 may be from R−NH2 rocking, the medium strength bands at 1118 cm−1 could be assigned to the νCX stretch and 1217 cm−1 could be assigned to the ν(C−O) stretch. With the reaction continuing further, other new Raman bands were observed and quickly became much stronger at 1071, 1242, and 1288 cm−1, which may be attributed to −COOH.30 This could be considered as the result of the depalladation and final formation of the carbonylation product of carboxyl-contained 4-aminoN,N-dimethylbenzylamine with much higher SERS responsiveness. The depalladation process was also observed in TEM images, which showed palladium black formed after the carbonylation reaction (Figure 1B). The comparison of the X-ray photoelectron spectroscopy (XPS) spectrum of AuNP/PC before and after incubation with saturated CO solution further verified the reaction on the surface of the nanosensors (Figure 1C). The XPS spectra of AuNP/PC were found changing after incubation, especially in the Cl 2p and Pd 3d spectra, which indicates that AuNP/PC reacted with CO and that the Cl group was consequently removed with the production of Pd(0). Moreover, mass spectrometry analysis of the independC


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Figure 2. SERS detection of CO in solution using AuNP/PC nanosensors. (A) SERS spectra of AuNP/PC after reaction with varying concentrations of CO. (B) Plots of ratiometric peak intensities versus logarithmic CORMs concentration based on I1032/I1454, I1118/I1454, and I1242/I1454. (C) SERS spectra of AuNP/PC after reaction with various biological species. (D) SERS intensities of AuNP/PC at 1118 cm−1 based on the spectra in part C for related biological species. (E) SERS spectra of AuNP/PC after reaction with CO in the presence of different biological species. (F) SERS intensities of AuNP/PC at 1118 cm−1 based on the spectra in part E for mixtures of CO (5 μM) and indicated biological species (50 μM).

while bands at 1032, 1118, 1242, 1288, and 1358 cm−1 appeared and were enhanced with increasing CORMs concentration. Notably, the band at 1454 cm−1 probably associated with the C−NH2−Au bond is almost unchanged in the carbonylation, indicating it could be used as the ratiometric peak. Moreover, as displayed in Figure 2B, there was an approximately linear relationship between the ratiometric peak intensities of I1032/I1454 (I1118/I1454 and I1242/I1454) and the logarithmic concentration of CORMs from 0.5 μM to 50 μM with a detection limit of approximately 0.34 μM (upon I1032/ I1454), 0.36 μM (upon I1118/I1454), and 0.42 μM (upon I1242/ I1454) (at a signal-to-noise ratio of 3). Consequently, the prepared AuNP/PC demonstrated a great promise as SERS nanosensors for the quantification of CO with relatively higher sensitivity. The selectivity of the SERS nanosensors was further evaluated by testing other biologically relevant species (Figure 2C−F). There were no obvious changes in the SERS spectra of AuNP/PC in the presence of other gaseous molecules (Na2S, NO), reactive oxygen species (H2O2, NaOCl, O2•−, •OH, ONOO−, tert-butyl hydroperoxide (tBuOOH), ROO•), and prevalent biological molecules (GSH, CN−). Although some biomolecules could cause a few extra bands in the spectrum of the nanosensor, no bands appeared at 1032, 1118, and 1242 cm−1 in contrast to exposure to CO. However, once the SERS probes were incubated with CO combined with these biologically relevant species, the carbonylation-related SERS peaks at 1032, 1118, and 1242 cm−1 were found (Figure 2C−2F), indicating that these species do not influence CO detection significantly. This may be due to the fact that these possible interferences could not trigger the palladacycle carbonylation as CO did. Thus, owing to the specificity of the carbonylation reaction and the fingerprinting feature of SERS, the proposed SERS-based strategy demonstrates high selectivity for the detection of CO in physiological media. Detection of CO in Living Cells. Prompted by the obtained promising results, in vivo studies were further

ent reaction of PC with CO (Figures S8 and S9) showed that the carbonylation product was 2-carboxyl-4-amino-N,N-dimethylbenzylamine, confirming the findings of −COOH by the proposed SERS strategy. This compound with −COOH was the dominant carbonylation product in aqueous solution instead of the 2-methyl isoindolin-1-one or dealkylative amide products, which agrees with the previously reported reaction mechanism11 and could be attributable to a hydrolysis reaction of the latter while it was formed. In addition, the reaction properties with CO were compared between PC and AuNP/PC (Figures S10 and S11). PC assembled on AuNPs react with CO more rapidly than PC without AuNPs support. More than 15 h were required to complete the carbonylation reaction after CO was introduced to the reaction system containing PC, whereas less than 60 min was required to complete the same reaction using AuNP/PC. The adsorption of CO at the AuNPs-aqueous interface may play an important role in this rapid reaction process.31 With this adsorption effect, CO was enriched on the surface of AuNPs where PC were assembled; thus, the PC had more opportunities to react with CO. As a whole, an SERS response of PC carbonylation initiated by CO was observed on the developed SERS nanosensors, and the carbonylation process was accelerated with AuNPs, suggesting potential of the fabricated SERS nanosensors for CO detection. SERS Detection of CO in Solution Using AuNP/PC Nanosensors. Utilizing AuNP/PC nanosensors, the detection of CO in PBS at pH 7.4 and 37 °C was investigated, during which biologically compatible2,24 and SERS inactive (Supplementary Note 4, Figures S12−S14) CORMs were employed instead of the saturated CO solution to accurately produce equal CO in a PBS system. Investigation of the dose-dependent SERS response of the nanosensors to CO showed that the SERS spectra of AuNP/PC changed significantly after 30 min reaction with different concentrations of CORMs (Figure 2A).When 0.5 μM CORMs was introduced, the bands at 1173 and 1209 cm−1 disappeared, D


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assembled on the surface of AuNPs, which enabled the nanosensors to mediate carbonylation without hampering their SERS activity. This allowed for direct observation of the COtriggered carbonylation process of PC through SERS spectrum variations of AuNP/PC nanosensors. Moreover, probably benefiting from the adsorption of CO on AuNPs, the reaction time of palladacycle carbonylation was shortened significantly after assembling PC on AuNPs, which facilitated CO detection. We have shown that the presented SERS nanosensors can be applied to selectively detect intracellular CO. Owing to the specific features of the PC carbonylation and selective advantages of the SERS technique, the detection of CO in living cells was accomplished without interferences from other biological species. Additionally, the present nanosensors using simple AuNPs as SERS-active substrates respond to CO at the sub-micromolar level. This sensitivity may be greatly improved by replacing AuNPs with other nanostructures with much higher SERS activity to perform the detection of endogenous CO. These results indicated that the proposed SERS strategy provides a promising approach for the study of CO function in biological systems where highly selective and sensitive detection of intracellular CO is required.


conducted to assess the utility of AuNP/PC as SERS nanosensors for the detection of CO in living cells. HeLa cells and L02 prepared in culture dishes were incubated with a colloidal suspension of SERS nanosensors, and the bright-field images were measured after washing away the excess components with PBS (Figure S15A, Figure 3A). The

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Figure 3. SERS detection of CO in living cells. (A) Bright-field images of L02 cells after 4 h incubation with AuNP/PC. (B) DFM images of corresponding L02 cells containing AuNP/PC. (C−E) Detailed DFM images of L02 cells containing AuNP/PC. (F) SERS response of AuNP/PC to CO in L02 cells CO after addition of CORMs with different concentrations: 0.5 μM, 5 μM, and 30 μM. The assay mixture consisted of cells (2 × 106 cells/mL) and AuNP/PC (2 nM) in 0.01 M PBS buffer (pH = 7.4).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b01793. Synthesis detail and characterization data, the representative SEM image of AuNPs, density functional theory calculations, HPLC−MS analysis, cytotoxicity of the nanosensor, and data from the control sample (PDF)

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distribution of AuNP/PC was recorded with a dark-field microscope (DFM) and is shown in Figure S15B−E and Figure 3B−E, revealing that SERS nanosensors entered into cells and accumulated in the cytosol by means of endocytosis. With DFM images, we localized the positions of the SERS nanosensors and collected the corresponding SERS spectra (Figure 3F, Figure S15F). No SERS peak was collected without the addition of exogenous CO, probably because the SERS responses of PC and compositions in cells are too weak to generate SERS characteristic peaks. Nevertheless, a SERS band at 1032 cm−1 was observed in the presence of CORMs (0.5 μM), and more SERS bands at 1118 and 1242 cm−1 appeared and become stronger in the presence of higher concentrations of CORMs (5 μM, 30 μM). Meanwhile, there is a good reproducibility in ratiometric peak intensities I1032/I1454, I1118/ I1454, and I1242/I1454 (Figure S16). In addition, poison experiments are important for the 12 h observation of intracellular probes. The cytotoxicity of the SERS probe was assessed, and more than 90% cells remained viable compared with the control samples after incubating with AuNP/PC nanosensors up to 12 h (Supplementary Note 5, Figure S17). Thus, on the basis of observing palladacycle carbonylation, our SERS-based strategy could be applied to the detection of CO in living cells.

AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 021-64250032. E-mail: [email protected]. *E-mail: [email protected] Author Contributions †

Y.C. and D.-W.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Basic Research 973 Program (Grant 2013CB733700), the NSFC (Grants 21421004, 21125522, 21327807, 21575041), the Shanghai Pujiang Program Grant of China (Grant 12JC1403500), the Shanghai Municipal Natural Science Foundation (Grant 14ZR1410800), and the Fundamental Research Funds for the Central Universities (Grant WB1113005).

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CONCLUSIONS In summary, we have presented a novel SERS nanosensor for the detection of CO in solution based on the reaction of palladacycle carbonylation by rationally designing and preparing nanosensors of AuNP/PC. With the electrostatic interaction between AuNPs and amine groups, PC readily E


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Highly Selective Detection of Carbon Monoxide in Living Cells by

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