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Ultrasensitive SERS sensor of gaseous aldehydes as biomarkers of lung cancer on dendritic Ag nanocrystals Zhen Zhang, Wei Yu, Jing Wang, Dan Luo, Xuezhi Qiao, Xiaoyun Qin, and Tie Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b05117 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017
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Ultrasensitive SERS sensor of gaseous aldehydes as biomarkers of lung cancer on dendritic Ag nanocrystals Zhen Zhang,†,‡ Wei Yu†,‡, Jing Wang,†,‡ Dan Luo,†,‡ Xuezhi Qiao,†,‡ Xiaoyun Qin,†,‡ Tie Wang*,†,‡ †
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡
University of Chinese Academy of Sciences, Beijing 100049, China
ABSTRACT: Surface-enhanced Raman scattering (SERS) is expected as a technique that even theoretically detect chemicals at single molecule level by surface plasmon phenomena of noble metal nanostructures. Insensitivity of detecting Raman weak-intensity molecules and low adsorptivity of gaseous molecules on solid substrates are two main factors hindering the application of SERS in gas detectors. In this manuscript, we demonstrated an operational SERS strategy to detect gaseous Raman weak-intensity aldehydes that have been considered as biomarker of lung cancer for abnormal content was measured in volatile organic compounds (VOCs) of lung cancer patients. To enhance the adsorption of gaseous molecules, dendritic Ag nanocrystals mimicking structural feature (dendritic) of moth's antennae were formed, wherein the existence of numerous cavity traps in Ag dendritic nanocrystals prolonged reaction time of the gaseous molecules on the surface of solid surface through the "cavity-vortex" effect. By nucleophilic addition reaction with the Raman-active probe molecule p-aminothiophenol (4-ATP) pregrafted on dendritic Ag nanocrystals, the gaseous aldehyde molecules were sensitively captured to detect at ppb (parts per billion) level. Additionally, the sensitivity of this operational SERS strategy to detection the lung cancer biomarkers was not affected by the humidity, which represented a great potential in fast, easy, cost-effective and noninvasive recognition of lung malignancies.
Rapidly detection of chemical agents with high sensitivity in the gas phase is of significant importance for environmental monitoring, explosives detection and clinical diagnosis.1-7 Various analytical techniques, like gas chromatography-mass spectrometry,8 ion mobility spectrometry,9,10 selected ion flow tube mass spectrometry and chemical sensors11-13 have been employed to measure gaseous molecules. The challenge comes as how to detect low concentration of chemicals in the air containing diverse chemical compositions, which generally requires complex analytical procedures or time-consuming process, such as preliminary enrichments or preconcentration procedures.14-16 Because of the advantage of surface enhanced Raman scattering (SERS) in highly sensitively, specifically identifying chemical and biological compounds through their unique vibrational fingerprints, several SERS sensing schemes was then prevailing for gaseous molecules detection.17-19 For example, Lin et al.17 developed a programmable localized electrodynamic precipitation concept for detecting airborne species in the electrospray ionization system, wherein charged photoresist layer was designed to selectively concentrate the airborne targets on the nanostructured sensor. Yang et al.18 demonstrated a sensing platform to collect SERS signals of gas-phase mole-
cules via enrichment of a suitable liquid solvent on the nearly pinning-free substrate. Piorek et al.19 combined a microfluidic device with Raman spectrometer for SERS detection, where the gaseous molecules were flowed in the open microfluidic channel to react with aqueous phase of silver nanoparticles. The key points to apply SERS of gaseous molecule detection in a broad field can be attributed to two aspects, insensitively detecting Raman weak-intensity molecules and improving absorbability of gaseous molecules on SERS substrates. First, the current SERS detection was mainly confined in SERS-active molecule, resulting in detectable species was restricted. Many Raman weakintensity molecules enable to provide important information for human health. Typically, 42 volatile organic compounds (VOCs) which were elevated to levels between 10 and 100 ppb in lung cancer patients have been identified as the lung cancer biomarkers.20-22 These biomarkers are mainly Raman inactive molecules, and can be broadly separated into seven categories: hydrocarbons, alcohols, aldehydes, ketones, esters, nitriles and aromatic compounds.7,23 For the aldehydes, as its carcinogenicity, teratogenicity, mutagenicity and genetic toxicity, only 1.0 ppm (parts per million) of glyoxal can promote the cellaging process.24 Exhaled aldehydes reflect aspects of oxi-
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dative stress and tumor-specific tissue composition and metabolism, so certain aldehydes in exhaled breath have been identified as an important indicators of lung cancer.7,25 Second, the adsorption properties of gaseous molecules on solid substrates could be tailored by sculpting surface shape of the substrates.26 As gaseous molecules diffuse fast, they present quite distinct adsorption characteristics on a planar substrate compared to those on a rough system (Figure 1a and b). In nature, male moths can detect a single molecule of female hormone from miles away via their most developed sense of smell, which benefit from broad shape of feathery antennae to contact with the target molecular as the largest possibility, making them perfect scent receivers for gaseous molecules.27,28 Inspired by structural feature (dendritic) of moth's antennae, we here demonstrated a reliable SERS strategy by forming a dendritic Ag nanocrystal substrate to measure the typical lung cancer biomarkers of aldehydes. Due to numerous cavity traps of special structural characteristics, gaseous aldehydes flow through the dendritic surface to experience "cavityvortex" effect, increasing the interaction time between the gas and the surface.29,30 When the aldehydes bounded on dendritic Ag nanocrystals via nucleophilic addition reaction with the Raman-active probe molecule paminothiophenol (4-ATP), SERS signal of 4-ATP exhibits remarkable change to present a detection limit of ppb (parts per billion) level. This ability to sensitively detection of Raman weak-intensity gaseous aldehydes by SERS provided tremendous prospects for screening test at early stage of lung cancer.
RESULTS AND DISCUSSION Generated by the interaction between a laminar boundary layer and the cavity, air flow passes through dendritic surface to exhibit periodic oscillation with three-vortex structure.31-33 First, a clockwise rotated main vortex was generated inside the cavity, from the trailing edge over about the cavity length (Figure 1b). Second, a secondary, smaller, counter-rotating vortex close to the cavity leading edge was induced by the main-vortex. A large group of insects have such antennae as the main sensory organ to take good use of this "cavity-vortex" for smell and touch stimuli.27,28 As illustrated in Figure 1c, the antenna of the silkmoth Bombyx mori is compose of an obvious trunk, numerous approximately parallel side branches, and symmetrically distributed villus. The widespread tiny holes from the villus are directly connected to the nervous system to collect external gaseous molecules. The adjacent ramus created the cavity, enabling the inside flow periodically recirculate to efficiently slow down the airflow, enriching the collection and delivering analytes into the tiny holes (Figure S1).
Figure 1. (a, b) Sketches of the cavity flow dynamics. (c) The photo of silkmoth Bombyx mori's antennae. Inset: a silkmoth Bombyx mori. (d) Low and (e) high resolution scanning electron microscope (SEM) images of the dendritic Ag nanocrystals. Dendritic Ag nanocrystal possessing structural characteristics of analogous antennae was fabricated by a galvanic exchange synthesis approach (Figure 1d and e). The driving force to fabricate such metallic hierarchical dendrite is electrical potential difference between two metals.34 For instance, a salt solution (BX), where metal B has higher reduction potential, can be displaced by metal A when metal A has a lower reduction potential (A+BX→ AX+B, where X is an anion). In this system, commercial Cu foam and AgNO3 aqueous solution was chosen to preparation of Ag dendritic nanocrystals. As the standard electrode potential of Ag (E / 0.799 V is higher than that of Cu (E/ 0.337 V,35,36 the Ag+ is rapidly reduced to Ag atoms to get supersaturated Ag atoms at the initial of Cu foil immersed in AgNO3 solution. Asfabricated dendritic Ag nanocrystals have a highly symmetric structure, consisting of 5-60 µm trunks (0.5-2 µm in diameter) and 0.5-4 µm sidebranches (50-600 nm in diameter). These sidebranches parallel to each other and mostly have an angle of about 55-60° with the central trunk (Figure 2a,b). At absent of surfactants, Ag atoms aggregated to grow along the direction of the lowest surface energy for both of the trunk and sidebranches,37 which were proved by high-resolution transmission electron microscopy (HRTEM) images and X-ray diffraction (XRD) patterns. HRTEM images collected from the trunk (Figure 2c) and the branches (Figure S2) clearly display dspacing (111) of 0.24 nm, which are consistent with their corresponding selected-area electron diffraction (SAED) patterns (the inset of Figure 2c and Figure S2). The XRD
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patterns (Figure 2d) exhibit face-centered cubic (fcc) (JCPDS No. 65−2871) Ag crystal with the strongest intensity peak at (111) facet.34 The X-ray photoelectron spectroscopy (XPS) spectrum of the freshly Ag dendrites shows two binding energy peaks at 368.0 eV for Ag 3d5/2 and 374.1 eV for Ag 3d3/2, clearly revealing that the freshly Ag dendrites has a weak oxidation (Figure S3).38
Figure 2. (a, b) TEM images of as-fabricated dendritic Ag nanocrystals. (c) HRTEM image of the Ag trunk taken from the spot of (b). The interspacing of 0.24 reveals that the Ag trunk grows along {111} orientation. The inset displays corresponding SAED pattern diffracted from Ag [111] direction. (d) XRD pattern of the dendritic Ag nanocrystals. TEM images of (e) Ag microwires and (f) Ag nanowires. To demonstrate the "cavity-vortex" effect of the Ag dendrite to enhance adsorptivity, two parallel samples dominated by Ag (111) crystal facets were prepared (Figure 2e,f and Figure S2), including Ag microwires (with 0.5-2 µm in diameter) and Ag nanowires (about 100 nm in diameter) that have similar size of the trunk and branches of dendritic Ag nanocrystals, respectively. The specific surface areas of the dendritic Ag nanocrystals, Ag microwires, and Ag nanowires are 1.657 m2/g, 1.232 m2/g, and 9.642 m2/g, respectively (Figure S4).
Figure 3. Optical images of (a) dendritic Ag nanocrystals, (e) Ag microwires, and (i) Ag nanowires, as well as
their corresponded (b, f, j) fluorescence images. (c, g, k) Schemes illustrate the adsorption of gaseous molecules in images of (b), (f), (j), respectively. (d, h, l) Velocity magnitude and flow pattern gained from ANSYS 14.5 CFD software for (d) Ag dendrite, (h) Ag microwire, and (l) Ag nanowire shapes. Fluorescence-based imaging experiments provided a direct insight to compare adsorptive capability of gaseous molecules on dendritic Ag nanocrystals, Ag microwires, and Ag nanowires (Figure 3), which were carried out by testing the fluorescence of o-phthalaldehydes (OPA) under fluorescence microscopy (for more details see the Method section).17 If not consider morphology influence, the OPA should exhibit a similar affinity to (111) crystal facets of dendritic Ag nanocrystals, Ag microwires, and Ag nanowires. The intensity of fluorescence on these three samples displays significant difference. The strongest blue fluorescent signal on dendritic Ag nanocrystals indicates they have the most outstanding property in collecting the gaseous molecules because of morphological effect (Figure 3b and Figure S5). Comparatively, no detectable fluorescent signal was found on both of Ag microwires, and Ag nanowires (Figure 3e-l). The simulated diagram of laminar and disturbed flow for the Ag dendrite was generated and analyzed in ANSYS 14.5 CFD software. The result shows that when the gaseous OPA flows through the dendritic surface, the shear layer impinge on the downstream edge of the sidebranches, the airflow instantaneously feed back to the upstream at impingement (Figure 3d). The resulted feedback develops a strong flow recirculation inside the cavity to slow down the airflow, resulted in a low momentum of gaseous OPA. Therefore, a longer reaction time is allowed to absorb gaseous molecules on dendritic Ag nanocrystals. Additionally, the experimental results and theoretical simulation is consistent with the references, where the fluid flow through the cavity experiences "cavity-vortex" effect.[30-33] The resulted feedback develops a strong flow recirculation inside cavity to slow down the airflow, resulted in a low momentum of gaseous OPA. Therefore, a longer reaction time is allowed to absorb gaseous molecules on dendritic Ag nanocrystals. While, the gaseous OPA easily flow away from smooth silver microwire or nanowire surfaces (Figure 3h,l). Accordingly, the strongest Raman bands of the OPA were observed on dendritic Ag nanocrystals (Figure S5). As aldehyde molecules involving polar bonds (C=O) have comparatively small Raman cross section, they can not be sensitively detected by SERS, although Raman signals are significantly enhanced by freshly prepared Ag substrate. Here, Raman-active 4-ATP molecules act as a bridge to bond gaseous aldehydes (Figure 4a). The 4-ATP enables to absorb on dendritic Ag nanocrystals through the thiol group, leaving the amine group to interact with other functional molecules.39 To quantitatively evaluate the adsorption of the 4-ATP, SERS measurements were
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collected in function of exposing time in 100 ppb 4-ATP vapor from 5, 10, 20, 30, and 60 mins (Figure S6). The characteristic Raman peak at 1067 cm−1 started to clearly observe within the first 5 minutes. The peaks appeared at 1003, 1067, 1138, 1386, 1437, and 1571 cm-1 are in good agreement with previously reported experimental and theoretical results.40 With the exposing time t, the peak intensity at the 1003 cm-1 (C-C symmetric stretch ring breathing) increases quickly and begins to level off after 40 mins. The gaseous aldehydes are sensitively captured by the reaction with the amino group of 4-ATP pre-grafted on dendritic Ag nanocrystals, which are evidenced by the changing at the Raman peaks of vibration state. Typically, the Raman spectra of the 4-ATP on dendritic Ag nanocrystals before and after reacted with the benzaldehydes are represented in Figure 4b. After reaction time of 120 min within 2.0 ppb benzaldehydes vapor, a significant peak appeared at 1616 cm-1 for C=N stretching due to the crosslinking between the-NH2 group of 4-ATP and the CHO group of benzaldehydes.41 The strong benzene ring vibration (1065 cm-1) shifts to a higher frequency (1073 cm1 ), as the π electron conjugation between two phenyl rings through the C=N bridging group. The weak broad band at 1231 cm-1 has been assigned to the bend or deformation mode of the C-H and N-H.39,42 Visibly, the Raman intensity gradually increases with increasing the concentration of benzaldehydes flow to have a good linearity in the range of 2.0 ppb-20.0 ppm (Figure 4c). For the Raman detection unaffected by moisture, the sensitivity of this method to the gaseous aldehydes was hardly affected by the presence of water molecules (Figure S7). Furthermore, other aldehydes molecules like gaseous glyoxal, glutaraldehyde and phenylacetaldehyde can be well detected at the 2.0 ppb level (Figure 4d), and well distinguished with each other (Table S1). Ascribed to the imine reaction of the -NH2 group of 4ATP and the -CHO group, this method exhibits excellent selective detection of the aldehydes, which has been proved by examining other potential interfering lung cancer biomarkers, including hydrocarbons (n-hexane, cyclohexane), alcohols (methyl alcohol, ethyl alcohol), ketones (acetone, butanone), esters (butyl acetate), nitriles (acetonitrile), and aromatic compounds (methylbenzene). The intensity ratio of Δ(I1170/I1137) (the difference between the intensity ratio of the analyte and the 4-ATP) was plotted against different analytes (Figure 4e). In marked contrast, except for the aldehydes, the intensity ratio of I1170/I1137 for other analytes displayed no apparent changes with 4-ATP. Additionally, the special aldehydes can be selectively detected from the mixtures of compounds with the targeted aldehyde (Figure S8).
Figure 4. SERS detection of the gaseous aldehydes. (a) Schematic illustration of the strategy to detect aldehydes. (b) Typical Raman spectral of the 4-ATP on dendritic Ag nanocrystals before (black) and after (red) reacted with benzaldehydes. (c) The plot of the Raman intensity of C=N peaks at 1616 cm-1 with the concentration of the gaseous benzaldehydes. (d) Raman spectra of the glyoxal, glutaraldehyde, benzaldehyde, and phenylacetaldehyde at the concentrations of 2.0 ppb. (e) Selectively detection of the aldehyde to other potential interfering lung cancer biomarkers, including (1) the hydrocarbon (n-hexane, cyclohexane), (2) the alcohol (methyl alcohol, ethyl alcohol), (3) the ketone (acetone, butanone), (4) the ester (butyl acetate), (5) the nitrile (acetonitrile), and (6) the aromatic compound (methylben-zene).
CONCLUSION In summary, we have developed an innovative strategy to ultrasensitively detect SERS weak-intensity molecules by employing a bionic antennae structure, dendritic Ag nanocrystals. Benefitted from the numerous cavity traps created by the multi-branched structure, through the "cavity-vortex" effect, the dendritic Ag nanocrystals overcome a long-standing limitations that gaseous molecule difficultly absorb on solid substrates. Combined with the pregrafted Raman-active molecule of 4-ATP, the gaseous aldehydes can be sensitively captured to detect at ppb level. As a universal strategy, the detection of other Raman weak-intensity VOCs could also use this strategy by simply reacting with other appropriate Raman-active molecules. With the capability to selectively trap and sensitively SERS detection of trace gaseous aldehydes under high moisture conditions, this facile strategy has an observable potential in real-world applications, i.e., tremen-
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dous prospects for noninvasive recognition of lung malignancies.
(15) Kim, K. H.; Jahan, S. A.; Kabir, E. Trac-Trend Anal Chem 2012, 33, 1-8. (16) Boots, A. W.; van Berkel, J. J. B. N.; Dallinga, J. W.; Smolinska, A.; Wouters, E. F.; van Schooten, F. J. J Breath Res 2012, 6.
ASSOCIATED CONTENT Supporting Information Experimental procedure and additional information and data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This research was financially supported by the 1,000 Young Talents program, the National Natural Science Foundation of China (Grant Nos. 21422507, 21321003) and the Chinese Academy of Sciences.
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Ultrasensitive SERS sensor of gaseous aldehydes as biomarkers of lung cancer on dendritic Ag nanocrystals
Table of contents
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