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) superstructure 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 characteristic CV peaks were obvious even at a low concentration of 1010 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 possesses excellent practical reproducibility and represents a promising candidate for chemical and
biomedical inspection.
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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 1010 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
3.2 103 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 cm1 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 cm1. There
were also two clear, sharp peaks at 98 and 438 cm1, 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 cm1 could be attributed to the
acoustic phonon overtone with A1 symmetry [26,26a]. A peak
centred at 583 cm1 regarding the optical phonon confinement
mode could be attributed to the E1 (LO) phonon [25,26,26a]. Other
peaks e 333 and 383 cm1 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 cm1 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