Facile One-Step Photochemical Fabrication and Characterization of

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Facile One-Step Photochemical Fabrication and Characterization of an Ultrathin Gold-Decorated Single Glass Nanopipette Xiaolong Xu,† Haili He,†,‡ and Yongdong Jin*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: The inner surface of a conical glass nanopipette was modified with ultrathin gold film by a facile one-step photochemical approach, using HAuCl4 and ethanol as common reagents with the aid of UV irradiation. The method is simple, straightforward, time-saving, and environmentally friendly. The morphology and component of the as-prepared ultrathin gold film was thoroughly characterized by transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). The mechanism of the gold film growth was briefly discussed. Other small photochemical reagents with a hydroxy group, e.g., ethylene glycol, methanol, and glucose, may also work but with a different rate of reaction. The facile ultrathin gold decoration of a single glass nanopipette renders the glass nanopipette-based nanopore platform very easy for surface chemical modifications and potential sensing applications. The success of the gold decoration on the inner surface of the glass nanopore was further confirmed electrochemically by surface modification of a small thiol molecule (cysteine), and the pH (surface charge)-dependent ionic current rectification behaviors through the nanopore were investigated. Due to its facile preparation, the method and the Au-decorated glass nanopore would find promising and extended applications in ultrasensitive detection and biosensing.

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electrical detection with embedded electrodes,5 optical readout,21 optoelectronic control,22,23 and the applications of magnetic tweezers,24 field-effect,25,26 and pressure,27 was used in combination with the nanopore platform to discern the whole picture. Besides the above-mentioned tactics, some research groups devote their effort to the modification of nanopores.11,28−39 The advantages of the chemical way could be expected. First, the detection specificity can be introduced by tailoring functional molecules, e.g., DNA,32 aptamer,11,29 antibody,33 and molecular imprinted polymer. Second, it provides a possible way to study the interaction between (bio)molecules by attaching one molecule to the nanopore and leaving the other in the bath.38 Third, intelligent control of molecular and ionic transportation could be realized by attaching a stimuliresponsive molecule and exerting stimuli, such as pH, light, and temperature. As a member of artificial solid state nanopores, the glass nanopipette-based nanopore has its unique advantages, such as stiffness and durability (compared to natural protein-based nanopore structures), easily made with adjustable size and shape, and low cost. To circumvent the shortcoming of glass nanopores in surface modification, the introduction of gold film inside the glass nanopipette is therefore attractive and is one of

rtificial solid-state nanopores have recently gained more and more attention for their applications in the field of single-molecule studies.1−18 Among nanopore-based analytics, the resistive-pulse sensing is the most prevalent way to extract and deduce information. The prototype device of such method consists of two chambers connected by the nanopore. An applied voltage across the nanopore drives ions and charged targets or analytes through the nanosized pore, during which the ion current is recorded. When translocating through a nanopore, the targets or analytes partially occupy the nanopore orifice, which would usually result in a conductance decrease and a current decrease pulse (in some cases,19,20 conductance increased and a current increase pulse was recorded). Thus, the frequency, amplitude, duration of the current pluses, and profile or fingerprint of the current−time curve could be used to identify the target molecule or to deduce the translocation process and the interactions of the target molecules with the nanopore. However, usually the molecular information obtained by the nanopore analytics is much less than expected due to the relative difficulty in surface chemical modification and the lack of good control on the ultrafine nanometer scale of the systems, which are believed to be very crucial for modulating (and extracting) molecular interactions (information) between the translocation target and the nanopore. In other words, the major drawback that hinders the sensing application of such a promising and potent method is the deficiency in specificity and sensitivity. To make up for this shortcoming, an extra device design or technology, such as © XXXX American Chemical Society

Received: September 10, 2014 Accepted: February 20, 2015

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diameters of these nanopores were typically around 50 nm (48 ± 8 nm), which could be estimated by SEM measurement (Figure S1, Supporting Information). Photochemical Decoration of a Single Glass Nanopipette with Ultrathin Gold Film. The decoration of gold onto the inner surface of glass nanopipette was done by a photochemical approach (as schematically shown in Figure 1).

the most promising shortcuts to expand the application of glass nanopores due to the functionalization and the rich thiol-Au chemistry. Although techniques for building ultrathin gold films onto a plane glass surface, such as electrodeposition, electroless plating,40 ion sputtering deposition, electron beam evaporator, and so on, are mature, direct transplant of these techniques to a nanosized fine glass nanopipette is restricted. Since the glass nanopipette has a taper between the orifice and the main shaft, diffusion of the Au plating bath from inside the main shaft to the orifice is hindered there; the formation of Au films, especially ultrathin Au films, is uncontrollable (if reacting fast), and the reaction would mostly take place at the taper zone. Then, the glass nanopipette would be blocked during its modification. Thus, methods that can create heterogeneous nucleation sites on the inner wall of the glass nanopipette and with slow reaction kinetics are very crucial and will help in the success of the Au modification process. Very recently, we have developed a chemical method to prepare ultrathin Au-decorated glass nanopores via a mild surface-confined enzymatic reaction.41 In this study, we report a much simpler, one-step photochemical method to prepare ultrathin Au-decorated single glass nanopores. HAuCl4 and ethanol were used as common reagents to slowly grow ultrathin gold films on an inner wall of the glass nanopipette under UV irradiation. Since the inner wall of glass (quartz) nanopipette (with typical isoelectric point of 1−4 depending on the ingredients of the silicate glass used) would be negatively charged (as a result of deprotonation of quartz) at the reaction pH (∼4.5), the homogeneously negatively charged sites or defects on the glass wall act as the nucleation sites during the photochemical process.42 The successful preparation of the ultrathin gold film was confirmed thoroughly by electrochemistry, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM)/TEM-mapping characterizations. The mechanism of the gold film growth was briefly discussed. Control experiments show that other small photochemical reagents with a hydroxy group, e.g., ethylene glycol, methanol, and glucose, also work but with poorer quality control. As a model molecule, cysteine has been used for surface modification of the Au-decorated nanopore surface. Different ion current rectification (ICR) behaviors were observed by tuning the charge state of the self-assembled cysteine monolayers, further confirming the success of Au decoration. The method may extend and push glass nanopipette-based nanopore platforms for potential practical sensing applications.

Figure 1. Schematic representation of the photochemical preparation of ultrathin gold-decorated single glass nanopipette. This scheme is not drawn to scale.

First, a given amount of C2H5OH was mixed with HAuCl4 solution. This solution was backfilled into the glass nanopipette. Then, the nanopipette was placed directly under the UV light source (254 nm, 8 W, Model ZF-5, Jiapeng Instrument Co., Ltd., Shanghai, China) for a given time at room temperature. Typically, the ratio of C2H5OH/HAuCl4 solution (8 mM) was 2:3 (v/v). After UV light irradiation, the nanopipette was rinsed with copious C2H5OH and water, dried at room temperature, and annealed at 100 °C for 1 h. Control experiments were also performed using ethylene glycol, methanol, and glucose (0.5M) instead of ethanol. Solutions were prepared with deionized water. Chemical Modification of a Gold-Decorated Single Glass Nanopipette. The ultrathin gold-decorated single glass nanopipettes were filled with 20 mM cysteine aqueous solution or 10 mM cysteamine ethanol solution, incubated for 30 min at room temperature, and then rinsed thoroughly with water or ethanol/water, respectively. Characterizations. All ionic current measurements were performed using a CHI 832D electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). The current was collected by applying voltage between the Ag/AgCl electrode inside the nanopipette and the external Ag/AgCl reference electrode facing the nanopore orifice. UV−vis absorption spectra were recorded by a CARY 500 UV−vis-NIR Varian spectrophotometer using a 1 cm path length quartz cell at room temperature. 100 μL of the reaction mixture was sampled at a given time, diluted by 75× for UV−vis measurement. The SEM images of the nanopipettes were taken with a XL30 fieldemission scanning electron microscope at an accelerating voltage of 15 kV. To improve image quality, the nanopipettes were coated with a thin layer of gold using an ion beam coater. TEM and high-angle annular dark-field scanning TEM (HAADF-STEM) were carried out by using a FEI TECNAI F20 EM with an accelerating voltage of 200 kV equipped with an energy dispersive spectrometer. The tip (∼2 mm) of the nanopipette was cut off and transferred to a folding grid for TEM imaging, an energy-dispersive X-ray (EDX) spectrum, and elemental mappings. XPS measurements were performed using a Thermo ESCALAB VG Scientific 250 with monochromatized Al Ka excitation. The resolution was 0.05 eV, and the binding energies were calibrated with C 1s (284.6 eV).

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EXPERIMENTAL SECTION Fabrication of Glass Capillary-Based Nanopores. Quartz glass capillaries (QF100-70-10, Sutter Instrument Co.) were used all through the experiments. Their nominal outer and inner diameters were 1.0 and 0.7 mm, respectively. All glass capillaries used in the experiments were thoroughly cleaned by immersing in freshly prepared piranha solution (3:1 98% H2SO4/30% H2O2) for ∼2 h to remove organic impurities. (Caution: piranha solution is a powerf ul oxidizing agent and reacts violently with organic compounds. It should be handled with extreme care.) The capillaries were rinsed thoroughly with deionized water and vacuum dried at 80 °C prior to use. The glass nanopores were then fabricated by using a CO2-laseractuated pipet puller (model P-2000, Sutter Instruments Co.) with a one-line program containing the following parameters:34 Heat = 700, Fil = 3, Vel = 40, Del = 175, Pull = 190. The inner B

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RESULTS AND DISCUSSION Fabrication and Characterization of an Ultrathin AuDecorated Single Glass Nanopipette and Nanopores. The glass nanopipettes used in this study had an inner diameter of about 50 nm (typically 48 ± 8 nm), which were estimated from steady-state voltammetry of ion transfer across the interface between two immiscible electrolyte solutions (ITIES) formed at the tip of the nanopipette. Not only the nanopore diameter but also the cone-angle of the nanopipette could be extracted from this electrochemical method.19 The results were comparable with those determined by SEM measurement (Figure S1, Supporting Information). The success of gold deposition onto nanopipettes’ tips could be revealed from the changes in their corresponding ionic current rectification (ICR) behaviors of the nanopore during the process. The ICR is observed as asymmetric current− voltage (I−V) curves, with the currents for one voltage polarity higher than that for the same absolute value of voltage but of opposite polarity. The magnitude of ICR is typically quantified by the rectification coefficient, r. The rectification coefficient is measured at a specific potential difference, and it is defined as the ratio between the average currents at opposite polarities (i.e., r = |I−/I+|, in this work, I+ and I− are the currents at 500 and −500 mV, respectively). The rectification coefficient of the nanopore increased from 5.75 to 8.48 after gold deposition (Figure 2). It was reported that the ICR behaviors of conicalFigure 3. (a) A typical TEM image of an ultrathin Au-coated single glass nanopipette. (b) EDX spectrum of the ultrathin Au-decorated single glass nanopipettes. Scale bar: 50 nm.

middle of the pipet is observed in situ to be caused by slight melting and collapse of the ultrathin Au films under electron beam irradiation. The EDX spectrum of the sample exhibited distinct peaks of the Au element (Figure 3b). The chemical nature and uniformity of the deposited ultrathin Au films was further confirmed by TEM elemental mapping (Figure 4). Note that the contour profile of the Au element distribution was in accordance with the shape of the nanopipette, which is a strong indication of homogeneous Au formation. The as-prepared glass nanopipette and ultrathin Au nanofilms were found to be robust for low and medium magnification TEM measurements but cannot survive from the strong electron beam irradiation

Figure 2. I−V curves of a single glass nanopipette-based nanopore recorded in 0.01 M KCl before (solid line) and after (dot line) the photochemical formation of an ultrathin Au film. Inset: Digital camera images of the nanopipettes before (right) and after (left) the Au decoration. The I−V curves were recorded in 0.01 M KCl (buffered with 10 mM HEPES, pH 7.0). Scan rate, 50 mV/s.

shaped nanopores were strongly affected by the surface charge of the nanopore.43,44 This increase in the rectification ratio is due to the strong adsorption of Cl− to Au, which brings the nanopore surface excess negative charges after the Au decoration.43,45 TEM characterizations provided direct and concrete evidence of the success of gold modification to nanopipettes. Figure 3a shows a typical TEM image of the sharp tip of the asprepared ultrathin Au decorated glass nanopipette. Instead of formation of big AuNPs or chunks, the inner wall of the nanopipette tip was homogeneously covered by an ultrathin Au film, as revealed by contrast homogeneity and some transparency of the wall remaining. The bubble like feature in the

Figure 4. (a) Dark-field TEM image of an Au-decorated glass nanopipette. (b) The corresponding TEM elemental mapping (determined by X-rays given off as electrons return to the L (Au-L) and M (Au-M) electron shell of gold) of the Au-decorated glass nanopipette at the side wall. Scale bar: 200 nm. C

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shows that the binding energy of Au 4f7/2 is located at 84.05 eV (Figure S5, Supporting Information) which is in accordance with Au(0).47 A control experiment was also conducted using ethylene glycol, methanol, and glucose (0.5 M) as photochemical reductant agents instead of ethanol, and similar UV− vis absorbance spectra were obtained (Figure S6, Supporting Information). Those reductant agents also work but with poorer quality control in Au film formation than that with ethanol. Generally the photochemical synthesis of colloidal nanoparticles has a two-step process,48−51 i.e., homogeneous nucleation and the successive growth of the particles. In our case, the heterogeneous nucleation and growth are of great importance since the reaction is conducted on the glass surface (rather than in solution). Note that the isoelectric point of the silicate glass surface is about 1−444 depending on ingredients of the silicate glass used and the glass wall would be negatively charged under the synthesis conditions. Therefore, as described with more detail in the Suplementary Information, the negatively charged sites or defects on the glass wall could preferentially adsorb precursor ions and acted as heterogeneous nucleation sites.42,52 In the first step, metal ions in solution are photochemically reduced to atoms and thus produced an agglomerate to form small clusters both in solution and on the glass wall. These nanoclusters then act as nucleation centers and catalyze the reduction process of the remaining metal ions present in the plating bath, leaving gold films on glass wall and particles in solution (precipitated upon its growth). TEM images in Figure 3 and Figure S3, Supporting Information, supported this hypothesis. As proven by our experimental results, though having a weak adhesion to glass surfaces, the wet-chemically prepared thin gold films on the inner wall of the conical glass nanopore were surprisingly quite stable and reproducible (Figure S7, Supporting Information) and survive the experiments in solution environments (cf. Figure S8, Supporting Information). Several factors may contribute to the relative “strong” adhesion and stability of the Au thin films: First, the charged sites on the glass surface, which act as nucleation sites for Au thin film formation, provide a weak interaction (van der Waals force). Second, the solution-processed Au thin films, when used in solution environments, can effectively escape the wetting damage that can usually lead vacuum prepared Au films to delaminate or desorb from the glass surface when there are no precautions taken to prevent it. Third, the wet-chemically prepared Au nanofilms are physically confined and supported within the inner wall of the nanosized conical glass nanopores, further rendering them stable for use in solution. Further work is needed to improve the adhesion and hence the stability of the Au thin films for robust practical applications. Switchable Ion Current Rectification Properties of the Cysteine-Modified Gold Nanopore. The photochemically grown ultrathin Au nanofilm on the inner wall of the glass nanopipette provides a superior surface (than the glass surface) for further facile chemical functionalization, e.g., using thiol molecules. In order to verify the feasibility of thiol molecule attachment to as-prepared gold film, we further investigate ICR behaviors of the Cys-modified gold nanopores (Figure 6). Under acidic conditions (pH 3.0), the ICR coefficient was 0.93, which is a positive current rectification. This is because the −NH2 groups of the self-assembled Cys are protonated and the −COOH groups are neutral in a solution environment below its isoelectric point (pI = 5.07). Thus, the overall net charge of

when performing high magnification TEM measurements. As is clearly shown in Figure S2, Supporting Information (and Figure 4a), after irradiation with a high energy electron beam (at high magnification and when performing elemental mapping), both the glass nanotip and the decorated ultrathin Au nanofilms melt and deform into incomplete structures. Due to the difficulty in measuring the thickness of Au thin nanofilms that deposited on the inner wall of the glass nanopipettes by TEM,41 the accurate thickness of the thin Au layers and the relationship between the coating thickness and irradiation time are difficult to obtain. However, we speculated that the Au thin films were growing over irradiation time to some extent based on UV−vis absorbance spectra of the reaction mixture before and after UV irradiation (Figure 5), and Au thin nanofilms with an estimated thickness of ∼2−4 nm can be reproducibly prepared by the typical preparation.

Figure 5. UV−vis absorption of the reaction mixture (supernatant, diluted by 75×) at a given time (0, 0.5, 1, 2, and 4 h, respectively) with and without (dk-4 h) UV irradiation.

Mechanism of Gold Film Formation. As shown in Figure 5, the UV−vis absorbance spectra of the reaction mixture before and after UV irradiation were monitored. Before UV irradiation, the solution displayed a strong absorption peak at 227 nm and a shoulder at 288 nm, both of which were due to the ligand-to-metal charge transfer (LMCT) bands of [AuCl4]− ions between chloric ligands and gold.46 These LMCT bands showed no apparent decreasing without UV irradiation but decreased drastically with increasing UV irradiation time, suggesting that the Au(III) in [AuCl4]− was reduced into the metallic state under UV irradiation (the possible mechanisms are described with more detail in the Supporting Information). Unlike that of colloidal AuNP synthesis, there is no plasmon band centered at ∼508−550 nm, indicating that there was no AuNPs left in the supernatant of the solution phase during the early stage of the photochemical process. Experimentally, we found that a 2 h UV irradiation time is ideal for obtaining highquality ultrathin Au nanofilms. When the irradiation time was extended to longer than 4 h, some of the Au nuclei in the solution grew, precipitated, and finally blocked off the sharp tip of the nanopipette (Figure S3, Supporting Information). We have also conducted control photochemical experiments using planar glass slides instead of conical glass nanopipettes. The SEM image in Figure S4, Supporting Information, shows clearly that a 4 h continuous UV irradiation results in the subsequently fast solution growth of colloidal Au particles with a size up to a few micrometers. The X-ray photoelectron spectroscopy (XPS) D

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pH 5.2, 7.4, and 9.4, respectively (Figure S10, Supporting Information). When the different ICR behaviors of the Au nanopore with and without cysteine attachment were compared, we inferred that other thiol molecules could also be attached onto the gold surface via Au−S chemistry, and the property (e.g., positive current rectification) of the nanopipettebased nanopore could be finely tuned chemically.

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CONCLUSION In conclusion, we demonstrate a facile one-step photochemical way to prepare ultrathin gold films on the inner surface of a single glass nanopipette. The method is simple, straightforward, time-saving, and environmentally friendly. HAuCl4 and ethanol were used as common reagents to grow ultrathin gold film with the aid of UV irradiation. The successful preparation of such gold film was thoroughly confirmed by combined electrochemistry, XPS, and TEM/TEM-mapping characterizations. The mechanism of the gold film formation was discussed briefly. Other small photochemical reagents with a hydroxy group, e.g., ethylene glycol, methanol, and glucose, may also work but with different rates of reaction. Due to the facile preparation and high quality of the ultrathin Au films, along with the rich Au−S chemistry available for versatile surface modification to improve both specificity and functionality, the method and the Au-decorated glass nanopore platform would find promising and extended practical applications in sensitive detection and biosensing.

Figure 6. I−V curves (n = 3) of Cys-modified gold nanopipette nanopore at different pHs. The I−V curves were recorded in 0.01 M KCl (buffered with 10 mM HEPES). Scan rate, 50 mV/s.

the self-assembled Cys is positive. Meanwhile, the nanopipette would reject cations, transport anions, and therefore show a positive current rectification. When solution pH increases, the current rectification reversed to the negative direction and the ICR coefficient increased. This is because the −COOH groups were deprotonated to negatively charged −COO− and the −NH2 groups were uncharged. Thus, the overall net charge of Cys is negative, and the corresponding ICR coefficient changes to 6.81, 8.56, and 11.56 gradually with pH changing to 5.2, 7.4, and 9.4, respectively. Note that the absorption of Cl− ion onto the gold film also contributes a negative charge, which would partially neutralize the positive charge (pH 3.0) and lessen positive current rectification.44 The results are quite reproducible. Figure S9, Supporting Information, shows ICR behaviors of another Cys-modified nanopipette displaying a similar tendency with slight current differences. In order to obtain a convincing result of positive rectification, cysteamine (mercaptoethylammonium), an aminated thiol molecule with one −NH2 and one −SH group, has been introduced to gold film instead of cysteine. Cysteamine (pKa = 8.23) is protonated in a solution environment below its pKa value (10 mM PBS, pH 7.0) and positively charged. Thus, a clear positive current rectification with an ICR coefficient of 0.19 was obtained (Figure 7). Under the same pH environment, Au-decorated glass nanopore without Cys modification showed no positive current rectification at pH 3.0 and less negative current rectification with an ICR coefficient of 3.01, 5.77, and 9.63 for

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ASSOCIATED CONTENT

S Supporting Information *

Discussion of the photochemical reduction mechanism, SEM and TEM images of glass nanopipettes, XPS spectra of control gold film on glass slide, and UV−vis absorptions of control reaction mixtures with UV irradiation recorded with different time periods. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 431 85262661. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant Nos. 21305136 and 21175125), the Hundred Talents Program of the Chinese Academy of Sciences, and the State Key Laboratory of Electroanalytical Chemistry (Grant No. 110000R387).

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DOI: 10.1021/ac5034165 Anal. Chem. XXXX, XXX, XXX−XXX