In situ synthesis of hybrid zinc oxide-silver nanoparticle arrays as a powerful active platform for surface-enhanced Raman scattering detection

lve ha Tieu Tu Doanh , Nguyen Van Hieu , Ton Nu Quynh Trang , Vu Thi Hanh Thu y, 700000, Viet Nam a r t i c l e i n f o Article history: Received 29 December 2020 Received in revised form 12 March 2021 Accepted 24 March 2021 nitoring, opening diverse fields at uilt from flexible ced Raman signals Raman scattering lyse with normal surface plasmon resonances (LSPR) of the plasmonic nanostructure and a chemical mechanism (CM) related to the charge transfer process between absorbedmolecules and a SERS active substrate. A superior SERS active platform typically has an enhanced EM based on a high density of hotspots distributed over a large area. In addition, the charge transfer process between SERS active plat- forms and probe molecules should have good uniformity and be * Corresponding author. Faculty of Physics and Physics Engineering, University of Science, Ho Chi Minh City, 700000, Viet Nam. ** Corresponding author. Faculty of Physics and Physics Engineering, University of Science, Ho Chi Minh City, 700000, Viet Nam. E-mail addresses: tnqtrang@hcmus.edu.vn (T.N. Quynh Trang), vththu@hcmus. edu.vn (V.T. Hanh Thu). Contents lists available at ScienceDirect Journal of Science: Advanc journal homepage: www.el Journal of Science: Advanced Materials and Devices 6 (2021) 379e389Peer review under responsibility of Vietnam National University, Hanoi.fingerprint sensing. These features can be exploited in biomedical Raman spectroscopy [4]. A prominent enhancement in SERS would involve an electromagnetic mechanism (EM) induced by localised1. Introduction Surface-enhanced Raman scattering (SERS) is a promising spectroscopic tool and has garnered extensive attention because of its attractive features, including rapid, nondestructive examination; ultra-sensitive, label-free detection; and spectral characteristics of diagnostics, food safety, and environmental mo unprecedented opportunities for inspection in trace levels [1e3]. Using substrates that are b materials, SERS can provide significantly enhan and superior advantages for analytes with small cross-sections that would not be easy to anaThis is an open access article under the CC BY-NC-ND license ( nc-nd/4.0/).Available online 3 April 2021 Keywords: 3D-ZnONF arrays SERS Plasmonic Charge-transfer interaction Selectivityhttps://doi.org/10.1016/j.jsamd.2021.03.007 2468-2179/© 2021 The Authors. Publishing services b license ( b s t r a c t Micro-structured molecular semiconductor film-based surface-enhanced Raman scattering (SERS) probes are an important analytical tool for both fundamental and technological research. This study proposed a novel zinc oxide (ZnO)-based, three-dimensional (3D) semiconductor nanoflower (NF) su- perstructure probe with unique physicochemical properties, including engineered hotspots allowing an arrangement of metallic nanoparticles (NPs), as a means to analyse target molecules. By changing the size, high-density hotspots distributed throughout the nanopetal-like ‘nanorods’ of ZnO supports. When used to analyse crystal violet (CV), there were synergistic effects of silver (Ag), ZnO, and CV molecules in the synthesised ZnONFs@Ag-CV SERS system. The SERS results revealed that the plasmonic surfaces of the self-assembled hotspots on the 3D ZnO superstructures provided effective molecular interactions between the ZnONFs@Ag platform and the Raman probe molecule. These interactions influenced the configuration and detection performance of SERS. Moreover, the performance was closely associated with enhancement of the electromagnetic mechanism and the charge transfer contribution in the platform between the semiconductor, metallic NPs, and the analyte molecules. As a result, the charac- teristic CV peaks were obvious even at a low concentration of 10
10 M. In a mixture of two probes, the ZnONFs@Ag chip provided an outstanding selectivity in the quantitative and qualitative evaluation of each target molecule at low concentrations. The synthesised 3D ZnONFs@Ag heterostructure chip pos- sesses excellent practical reproducibility and represents a promising candidate for chemical and biomedical inspection. © 2021 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.Faculty of Physics and Physics Engineering, U b Vietnam National University, Ho Chi Minh Cita niversity of Science, Ho Chi Minh City, 700000, Viet NamOriginal Article In situ synthesis of hybrid zinc oxide-si powerful active platform for surface-en detection a, b by Elsevier B.V. on behalf of Vietnam d/4.0/).r nanoparticle arrays as a nced Raman scattering a, b, ** a, b, * ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY-NC-ND T.T. Doanh, N. Van Hieu, T.N. Quynh Trang et al. Journal of Science: Advanced Materials and Devices 6 (2021) 379e389cost-effective, reproducible, and renewable. These factors are crit- ical in the quantitative and qualitative evaluation of a SERS plat- form for practical applications [5]. In previous studies, there has been an emphasis on SERS active platforms that use a noble metal, because this design provides a high level of SERS enhancement and uniformity via the EM factor. It is well known that enhancing EM fields with regard to plasmon excitation provided by metal particles can be achieved at nano- gaps between two or more closely packed metal nanoparticles (NPs), so-called hotspots. These areas can be generated between particles as well as along a flat metaledielectric interface that supports plasmon resonances [6,7]. The noble metals such as sil- ver (Ag) and gold (Au) have been decisively proved to contribute to extremely high SERS performance and have been widely investigated for the preparation of SERS substrates and sensing devices. However, the efficacy of using noble metals for SERS platforms can be easily influenced by potential instability due to easy aggregation and oxidation, indicating that they may not provide optical stability within a reasonable shelf life. While the noble metals have played a significant role in the scientific and technical development in the SERS field, the reproducibility and renewability of SERS substrates based on noble metals is a great challenge. Therefore, their practical applications have been markedly hindered [8,9]. In fact, it was recently acknowledged that SERS active platforms that guarantee a reasonable distribu- tion of noble metal nanomaterials to improve sensitivity and reproducibility are still not technically feasible. To obtain the aforementioned characteristics, numerous efforts have been proposed to enhance the versatility of SERS [10e12]. Among these strategies, semiconductor substrates have become potential candidates to overcome the shortcomings of noble metals and have exhibited notable Raman enhancement. The improved Raman signals have been considered to originate from other reso- nant contributions such as charge transfer between the analytes and semiconductor substrate, excitonic biocompatibility, better chemical stability, and molecular resonance [13,14]. The efficient utilisation of semiconductor SERS substrates is based on a chemical interaction mechanism, which plays a significant role in further enhancing the SERS active platform behaviour, as well as better chemical stability, which offers opportunities to use SERS for measurements. Moreover, the construction of SERS active plat- forms that include composite substrates incorporating semi- conductor materials and noble metals could provide great reproducibility and renewability. These novel alternative strategies may help to overcome the drawbacks of using noblemetals because semiconductors have a self-cleaning capability due to their pho- tocatalytic activity [15,16]. Unfortunately, compared to noble metal SERS substrates (EF ~ 106), the EF (~103) and limit of detection (LOD) of semiconductor SERS active platforms are relatively low and may be considered a bottleneck in the engineering of the SERS active semiconductors. Indeed, they are far from being sufficient for practical applications. Therefore, the development of novel strate- gies to enhance further the Raman performance of semiconductor substrates with a high SERS EF has drawn widespread attention in recent years. Considering the extraordinary advantages of combining mo- lecular semiconductors and noble metals, a hybrid nanostructure composed of a semiconductor and plasmonic metal is believed to be a particularly appealing feature to enhance the performance of SERS systems. Indeed, the co-existence of the EM and the CM is responsible for enhancing Raman scattering. The hybrid would increase the number of hotspots by modulating the morphology of the platform. It would also allow charge transfer between the Raman probe molecules and the active substrate through380plasmonic excitation of the noble metal because of the localised surface plasmons excited by photons of incident light and the active semiconductor platform supports in the SERS system [9,17,18]. Among various semiconductors, zinc oxide (ZnO) has drawn substantial attention as a versatile SERS substrate because of its distinct physicochemical properties, including a self-cleaning capability for absorbed analytes under ultraviolet (UV) irradia- tion, high stability, high activity, and nontoxicity [19,20]. Regarding the semiconductor SERS substrates, ZnO engineered in different shapese rods, domes, hexagons, stars, and flowerse has been used to assemble the plasmonic materials to preprogramme plasmonic hotspots and control the formation of well-defined nanogaps. This change had led to prominent SERS enhancements. Of note, compared with other semiconductor-noble metal hybrids, a three- dimensional (3D) ZnO-noble metal hybrid demonstrated superior SERS behaviour. These enhancements could be attributed to the optical mode in the 3D shapes of the ZnO microcavity, providing a whispering gallery mode effect, as well as an improvement of lightematter interaction related to the total internal reflection [21,22]. After evaluating SERS performance, thewhole platformwas exposed to UV light, leading to the disintegration of analyte mol- ecules. This process simultaneously ensured signal amplification, renewability, and reproducible Raman signals of SERS active plat- forms [23,24]. In addition, flower-like 3D ZnO superstructures increased the specific surface area available for decoration with plasmonic materials; this feature dramatically enhanced Raman signals for SERS-based sensors. Based on the above information, in this study a facile preparation approach was used to generate flower-like 3D ZnO superstructure ar- rays for SERS-based sensors. The structures were grown by the hydro- thermalmethodonaluminium-dopedzinc oxide (AZO) glass substrate, and silver nanoparticles (AgNPs) were allowed to self-assemble on the surface via an induced photochemical reduction route to produce ar- rangementswith a high density of hotspots and appropriate nanogaps. The highly arranged ZnO nanoflower (NF) arrays produced a radial nanostructure; each nanopetal comprised ZnO shaped like a nanorod. This design increased the effective surface area onwhich AgNPs could be decorated and increased the possibility of creating plasmonic hot- spots and absorbingprobemolecules. TheAg@ZnOactive platformwas employed to detect directly various target analytes as the probe mole- cules at a low concentration. It exhibited an outstanding multicompo- nent detection ability based on dual-analyte detection. Furthermore, this design improved solid-state molecular packing, favouring highly efficient charge transfer/transport in structures and facilitatingefficient trappingofphotons toenhance further theSERSperformance.TheSERS substrates exhibited excellent reproducibility, with an average relative standard deviation (RSD) of <10%. Finally, based on the experimental results, a model was proposed to explain themechanism bywhich the incorporation of both 3D-ZnONFs and AgNPs enhance the SERS performance. 2. Experimental 2.1. Chemicals and materials Zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 99%, Merck), hexa- methylenetetramine (HMTA, C6H12N4, 99%, Sigma-Aldrich), the AZO target (99%, Singapore Advantech), hydrochloric acid (HCl, 38%, Sigma-Aldrich), silver nitrate (AgNO3, 99%, Merck), ethanol solution (C2H5OH, <99.5%, Merck), crystal violet (CV, C25H30ClN3, 99%, Merck), and rhodamine 6G dye (R6G, C28H31N2O3Cl, <95%, Merck) were used without further purification. Glass wafers were used to fabricate the SERS probe. Double-distilled water was used throughout the research. 2.2. Preparation of AZO seed layers AZO seed layers were deposited on glass slides using reactive direct current (DC) magnetron sputtering. Before the deposition, 1  1 cm glass substrates were washed by ultrasonication in an C2H5OH and HCl aqueous solution and dried with nitrogen flux. AZO seed layers were deposited at an operating pressure of 3.2  10
3 Torr at a sputtering power 60W under an argon (Ar) gas flow for 70 min on the glass substrate. 2.3. Preparation of the ZnONFs active platform The flower-like 3D hierarchical ZnO structures were synthesised by the hydrothermal method. In a typical process, the ZnONFs growth solutionwas prepared by dissolving 10 mMZn(NO3)2$6H2O and 10mMC6H12N4 in 50mL deionisedwater at room temperature. The obtainedmixturewas continuously stirred for 30min to form a homogeneous dispersion. Then, the above solution was transferred into an autoclave with a Teflon liner; the AZO substrates were placed vertically into the solution. Finally, the autoclave was heated at 90 C for 7 h. The solutionwas then cooled to room temperature. The ZnONFs active platform was thoroughly washed with droplets of double-distilled water to remove undesired impurities and dried under a nitrogen gas (N2) flow. 2.4. Decorating AgNPs on 3D ZnONFs superstructure arrays AgNPs were easily decorated on ZnONFs platforms using UV irradiation. Briefly, 5 mL deionised water was added to an aqueous solution containing 500 mL of various concentrations of AgNO3 (0.5, 1.0, or 1.5 wt%) and stirred for 3 min. Subsequently, the ZnONFs active platformwas immersed in the above solution and exposed to UV irradiation for 90 min. After the growth of 3D ZnONFs with AgNPs, the samples were carefully rinsed with double-distilled water and then dried by under an N2 flow. The ZnONFs@Ag plat- formwas developed on the AZO substrates via processes illustrated in Fig. S1 (see details in the Supporting information). 2.5. Material characterisation The characteristic morphologies and the elemental mapping distribution of the fabricated platforms were evaluated by a field- emission scanning electron microscopy (FE-SEM) system equip- ped with energy-dispersive X-ray spectroscopy (EDX) and a scan- ning transmission electron microscope (STEM) (Horiba). The crystalline structure of the prepared ZnONFs and ZnONFs@Ag platforms was characterised by X-ray diffraction (XRD) (D8, Advance, Bruker). Raman scattering spectra of the specimens were investigated using a Horiba XploRA PLUS Raman system. The presence of the element and binding energy of the core level electrons of functionalised SERS substrates were assessed by X-ray photoelectron spectroscopy (XPS) via a Thermo Scientific K-Alpha spectrophotometer (AXIS, Supra). 2.6. SERS measurements For Raman measurements, SERS substrates with an operating range of 200e2000 cm
1 were measured on a Horiba XploRA PLUS Raman system with an excitation laser of 532 nm. CV-ethanol or a mixture of CV þ R6G was dispersed onto flat SERS substrates. All Raman evaluations were done under similar experimental condi- tions. The spectra were collected using a 20 objective lens with a laser spot diameter of about 1 mm,150mW laser power, and 30 s for all acquisitions. To assess the behaviour of the platforms, CV- T.T. Doanh, N. Van Hieu, T.N. Quynh Trang et al.ethanol or a mixture of CV þ R6G was dropcasted onto flat SERS 381substrates and dried at ambient temperature for 2 h in the dark (Scheme 1). 3. Results and discussion The morphologies of the synthesised flower-like 3D ZnO and ZnONFs@Ag were characterised using SEM. From the magnified SEM images (Fig. S2), the platform showed many well-defined flower-like 3D microstructures with a homogenous distribution of numerous nanopetals that appear like small nanorods self- assembled into the 3D nanostructures. The diameter of the nano- petals fluctuated from the base to the tips, indicating that the nanopetals had hexagonally sharpened tips and broad bases. There was good crystallinity of the synthesised flower-like 3D ZnO. The thickness and the length of the nanopetals were determined to be 1e1.5 nm and ~300 nm to several mm, respectively. Each flower-like ZnO nanostructure comprises many nanopetals, or nanorods, which could play a prominent role in improving SERS signal re- sponses by improving photon scattering and promoting the transport pathways of charge because of the semiconductor nature. The wettability of ZnONFs was assessed by water contact-angle measurements (Fig. S3). The contact angle on a hydrophobic sur- face was ~92. This hydrophobicity is likely due to the formation of the hydrophobic surface through the 3D nanostructured surface morphology of the native material (ZnO). The more hydrophobic surface efficiently prevented the droplets from spreading and allowed them to maintain a spherical shape. The molecules in the droplet were concentrated and adsorbed on a small area by rapid solvent evaporation, resulting in an increase in the target molecule concentration. The Raman peaks of the nanostructured synthesised ZnO were evaluated by Raman spectroscopy. As shown in Fig. 1(a), Raman spectroscopy of the synthesised ZnONFs showed six outstanding peaks at 98, 203, 333, 383, 438, and 583 cm
1. There were also two clear, sharp peaks at 98 and 438 cm
1, which correspond to Elow2 and E high 2 Raman modes of ZnO, respectively. These characteristic peaks were attributed to the lattice vibrations of the hexagonal wurtzite structure of ZnO and oxygen vibration [25]. A weak peak centred 203 cm
1 could be attributed to the acoustic phonon overtone with A1 symmetry [26,26a]. A peak centred at 583 cm
1 regarding the optical phonon confinement mode could be attributed to the E1 (LO) phonon [25,26,26a]. Other peaks e 333 and 383 cm
1 e were indexed to 2E2(M) and E high 2
Elow2 , respectively; these were attributed to the presence of multi- phonon processes because of zone boundaries. The appearance of two broad peaks between 900 and 1300 cm
1 was likely related to second-order Raman modes of ZnO, corresponding to the A1(LO) and E1(LO), respectively [27]. To obtain a good understanding of the film's microstructural characteristics, the synthesised ZnONFs specimen was examined with XRD. As illustrated in Fig. 1(b), there were diffraction peaks at 2q values of 31, 35, 46, 56, 62, 66, and 68, corresponding to (100), (002) (101), (102), (110), (103), (200), (112), and (201) crystal planes of the ZnO in typical hexagonal wurtzite-type (JCPDS card No. 36-1451), respectively. The peak with the highest intensity, 2q ¼ 35, was dominant compared with the other peaks, indicating that the growth of ZnO was preferentially oriented along the c-axis perpendicular to the (002) facet. The elemental analysis of the ZnONFs substrate was examined by EDX (Fig. S4). It showed the presence of Zn and O without other external impurities. The EDX and XRD results were consistent. XPS characterisation was used to investigate the presence of the chemical states of each element and the composition of the hydrothermal