Aptamer based surface enhanced Raman scattering
detection of adenosine using various core sizes of
Au
–Ag core–shell nanoparticles†
Fu-Hsiang Koaand Yu-Cheng Chang*b
The present study synthesizes different sizes of Au nanoparticles (NPs) by adjusting the concentration of
citrate. This method produces Au NPs with uniform sizes that can be used to grow Au–Ag core–shell
NPs by the seeding growth method. We report the fabrication of monolayer Au NPs or Au–Ag core–
shell NPs through the self-assembly of NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modified
silicon substrate. The results indicate that the self-assembly time of Au NPs or Au–Ag core–shell NPs
plays a crucial role in determining the surface coverage of NPs on the silicon substrate. The appropriate
sizes of Au NPs or Au–Ag core–shell NPs were optimized to yield the greatest SERS effect in the
rhodamine B molecule. The substrates with self-assembled Au–Ag core–shell NPs can also be used to
detect adenosine by a structure-switch aptamer. The combination of Au–Ag core–shell NPs and
DNA-based adenosine sensors provides a facile, high enhancement, low detection limit (0.1 nM) and low cost
fabrication, which shall be of significant value for practical applications of other aptamer sensing systems.
1.
Introduction
Surface enhanced Raman spectroscopy (SERS) is a powerful vibrational spectroscopy technique with a highly sensitive and selective application that allows for detection of chemical or biological molecules in very low concentration, and provides rich structural information.1,2The enhancement effect is
nor-mally described by two enhancement mechanisms. One is a long-range electromagnetic (EM) effect that results from the enhancement of the local electromagneticeld due to surface plasmon resonance of nanoscale surface roughness features in the 10–200 nm range.3–6 The enhancement intensity is
deter-mined by many factors, such as sizes, shapes, composition, and analytic molecules adsorbed on the substrates.7,8 The other
mechanism is a chemical enhancement effect that results from the charge-transfer excitation of chemisorbed molecules.1,7
Since its discovery, SERS has been employed as a powerful tool
in the non-destructive and ultrasensitive detection of proteins, DNA, nucleic acids, glucose, and small molecules.9–12
Aptamers are oligonucleic acid or peptide molecules that can bind to a wide range of target molecules (proteins, drugs, and amino acids, etc.) with high specicity and selectivity.13,14Due to
their high selectivity, stability, versatile target binding, and easy regeneration capabilities over traditional antibodies, aptamers have received considerable research attention as recognition ligands in biological detections.15Nanomaterials have recently
emerged as ideal candidates enabling efficient signal trans-duction and amplication in aptasensors. The nanomaterial-assisted aptasensors display unprecedented advantages in sensing applications and have attracted signicant consider-ation in other multidisciplinary studies.12,16Optical detection
methods such as colorimetry,uorescence, and SERS are most widely adopted for signal harvesting in aptasensors because of typical and easy approach techniques.17SERS provides a high
sensitivity that is comparable withuorescence detection. SERS has several intrinsic advantages over uorometry such as ngerprints of analyses, excitation at any wavelength, alleviated photo-bleaching, narrow peak widths, and reproducibility.1,7
The combination of SERS and nanomaterial-assisted apta-sensors open up a whole new area for the application of biosensors.
The ability to develop SERS substrates with strong enhancement factors, low cost, good stability, and high repro-ducibility is still an important challenge.18Various approaches
have been developed to fabricate SERS substrates, but with limited success.19The success of SERS is highly dependent on
the interaction between adsorbed molecules and the surface of
aDepartment of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, 30010, Taiwan
bDepartment of Materials Science and Engineering, Feng Chia University, Taichung,
40724, Taiwan. E-mail: ychang0127@gmail.com; Fax: 4-24510014; Tel: +886-4-24517250 ext. 5348
† Electronic supplementary information (ESI) available: UV-vis spectra of Au NPs, EDS spectra of Au NP and Au–Ag core–shell NP, TEM images of Au–Ag core–shell NPs, dimension and self-assembly conditions of Au NPs and Au–Ag core–shell NPs, SERS intensity for rhodamine B on the different sizes of Au NPs, SERS intensity for TMR on the different core sizes of Au–Ag core–shell NPs. SERS spectra for TMR on the different sizes of Au NPs. SERS intensity for TMR on the biggest core sizes of Au–Ag core–shell NPs at the different concentration of adenosine. See DOI: 10.1039/c4ra02762k
Cite this: RSC Adv., 2014, 4, 26251
Received 28th March 2014 Accepted 6th June 2014 DOI: 10.1039/c4ra02762k www.rsc.org/advances
PAPER
Published on 18 June 2014. Downloaded by Aston University on 22/08/2014 14:32:23.
View Article Online
plasmonic nanostructures. SERS substrates commonly used gold (Au), silver (Ag), or copper (Cu).1,20In general, Au and Ag are
most oen used as SERS substrates because they are air stable materials rather than Cu.18 Compared to Ag, Au has lower
enhancement factors in visible light, but it is easier to prepare as monodisperse sols in a variety of diameters.21By combining
these two metals into a single entity, the catalytic properties can be further enhanced and the localized surface plasmon absorption can be varied continuously between the limits of the monometallic Au and Ag nanoparticles (NPs).22,23Recently, a
strategy for extracting optical constants of the core and/or the shell material of bimetallic Au–Ag NPs from their measured surface plasmon extinction spectra was reported.24 SERS
enhancement as a function of the layered Au–Ag NP composi-tion and the state of their aggregacomposi-tion was investigated using pyridine and other types of probe adsorbates.25 However,
understanding the effect of core sizes on SERS signals and the synthesis of large sizes of Au–Ag core–shell NPs without aggre-gation is still unclear.
The present study has successfully synthesized the different sizes of Au NPs by a modied Turkevich reaction system.26The
concentration of citrate plays an important role in growing the different sizes of Au NPs. The Au–Ag core–shell NPs with different sizes have been synthesized by the seeding growth method. The appropriate self-assembly time can be used to control the similar surface coverage of Au NPs or Au–Ag core– shell NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modi-ed silicon substrate. Au–Ag core–shell NPs can provide a greater SERS effect on the rhodamine B molecule rather than Au NPs. The silicon substrates with self-assembled Au–Ag core– shell NPs can also be used to detect adenosine by a structures-switch aptamer. The fabrication of DNA-based adenosine sensors is facile, of high reproducibility, and low detection limit (0.1 nM), which shall be advantageous in applications for other aptamer sensing systems.
2.
Experimental
2.1 Chemicals
Hydrogen tetrachloroaurate(III) trihydride, ethanol, hydrogen
peroxide, adenosine, and phosphate buffered saline (PBS) were purchased from SIGMA. Silver nitrate, trisodium citrate dihy-dride, and sulfuric acid were purchased from J. T. Baker. 3-Aminopropyltrimethoxysilane (APTMS), rhodamine B, and 3-mercapto-1-hexanol (MCH) were purchased from Alfa Aesar. The three types of deoxyribonucleic acid (DNA) were synthe-sized by MDBio, Inc. All chemicals and materials were of analytical grade and used as received. De-ionized water (>18 MU cm1) was used throughout the experiments.
2.2 Characterization
The morphology of nanostructures was examined with aeld emission scanning electron microscope (FESEM) using a JEOL JSM-6700F SEM operating at 10 kV accelerating voltage. A JEOL-2010 transmission electron microscope (TEM) operating at 200 kV was used to examine the microstructures. An electron
dispersive spectrometer (EDS) attached to the TEM was used to determine the composition of NPs. A Hitachi U-2900 UV-vis spectroscopy was employed to characterize the optical proper-ties of NPs. The Raman spectra were performed by Confocal Raman Microscope (HORIBA, LabRAM HR) at room tempera-ture in the backscattering conguration. The source light was He–Ne laser emitting at a wavelength of 632.8 nm.
2.3 Preparation of Au NPs
The different sizes of Au NPs were synthesized by adding 1 mL of a different concentration of trisodium citrate to 10 mL aqueous solution of boiling 1 mM hydrogen tetra-chloroaurate(III) trihydride under vigorous stirring. Aer the
appearance of a wine color, boiling and stirring were continued under reuxing for 10 min and then cooled at room temperature.
2.4 Preparation of Au–Ag core–shell NPs
The 2.5 mL Au NPs with different sizes were diluted to 8 mL with de-ionized water and reheated to boil, followed by adding 1 mL of a different concentration trisodium citrate under vigorous stirring. Next, 2 mL of 5 mM silver nitrate was added, and the boiling was continued for 10 min. The Au–Ag core–shell NPs solution was cooled and stored at room temperature.
2.5 Preparation of substrate
Si (100) wafers were cleaned in a boiling piranha solution (H2O2–concentrated H2SO4, 3 : 7, v/v) for 10 min, and then
rinsed with de-ionized water and ethanol. The substrates were immersed into an ethanol solution containing 5 mM APTMS and then heated under reux for 2 h. Next, the substrates were rinsed with ethanol and de-ionized water, and then dried under a N2purge. The Au NPs or Au–Ag core–shell NPs were deposited
onto the amino-terminated bonding by using an immersion method. Aer immersing the substrates in a solution of Au NPs or Au–Ag core–shell NPs for an appropriate time, the substrates were immediately rinsed with de-ionized water and dried under a N2purge.
2.6 Detection of adenosine
Fig. 1 depicts schematically the method used to fabricate the adenosine sensing platform. All the solutions were prepared with a PBS buffer. First, the hybridization of aptamer solution with DNA (1) (50-AGA GAA CCT GGG GGA GTA TTG CGG AGG AAG GT-30) and DNA (2) (30-TAMRA-TCT CTT GGA CCC-50, TMR-labelled DNA): the 500 mL of 1 mM DNA (1) was mixed with 500mL of 1 mM DNA (2) and heated in a water bath at 95C for 10 min. This solution was cooled at room temperature and stored at 4C. Second, the thiolated DNA (3) (50-SH-AGA GAA CCT GGG-30) was modied to substrate according to the liter-ature:2720mL of 1 mM DNA (3) was dropped onto the prepared
substrate for 12 h at 4C. Aer rinsing with PBS, the substrate was immersed under 1 mM of MCH for 5 min and thoroughly rinsed. Third, the adenosine solution was diluted to various concentrations ranging from 0.1 nM to 1mM with PBS, and then
mixed in an aptamer solution with DNA (2) double-stranded DNA (1) with a 1 : 1 ratio. 20mL of mixture was then dropped onto the prepared SERS substrate for 2 h. Finally, the sensing platform was washed with a PBS buffer and dried under a N2
purge. The signal intensity of the band at 1648 cm1 was recorded versus different concentrations of adenosine.
3.
Results and discussion
3.1 Evaluation of the different sizes of Au NPs
The optical properties of Au NPs solution measured by UV-vis spectroscopy are depicted in Fig. 2a. The UV-vis spectra of Au NPs solution appear sharp and narrow. The maximum peaks are at 529, 524, 521, and 519 nm, respectively. The wavelength of maximum absorbance was decreased with the increase in concentration of citrate, as shown in Fig. 2b. The inset photos are depicted from le to right with the color change from dark wine to wine, and the concentrations of citrate at 15.5, 19.4, 25.9, and 38.8 mM, respectively. The blue shi is ascribed to the quantum connement effect arising from the reduced sizes of the Au NPs. The role of the citrate in the growth process of Au
NPs is not only reducing Au3+to Au, but also controlling the sizes of the Au NPs. If the concentration of citrate is lower than 15.5 mM, the sizes of Au NPs appear non-uniform. The UV-vis spectra (Fig. S1†) exhibit a broad peak, which is attributed to the lower concentration of citrate being unable to provide for the inhibition of aggregation. The appropriate concentration of citrate can be exploited to control the different sizes of Au NPs. The sizes of Au NPs were determined by transmission elec-tron microscopy. The variation of concentration of citrate was found to affect the sizes of Au NPs. Fig. 2c–f show TEM images of Au NPs with spherical geometry, which were grown in citrate concentrations of 15.5, 19.4, 25.9, and 38.8 mM, respectively. The diameters of the Au NPs are 29.5, 23.4, 18.7, and 13.1 nm, respectively. The high concentration of citrate may induce the reduction in the sizes of Au NPs at the same Au precursors. On the other hand, varying the concentration of citrate led to signicant changes in the sizes of Au NPs. The chemical composition of the NPs was analyzed by energy-dispersive spectroscopy (EDS). The EDS spectrum (Fig. S2†) reveals the composition of Au NP with only one gold component (Cu is from the TEM grid). Appropriate amounts of citrate led to the growth of Au NPs with different sizes.
3.2 Evaluation of the different core sizes of Au–Ag core–shell NPs
Recently, the seeding growth approach based on the temporal separation of nucleation and growth processes is considered to be a very efficient method in controlling the bimetal core–shell NPs size and shape precisely.28 Herein, the seeding growth
method can be used to synthesize Au–Ag core–shell NPs on the different sizes of Au NPs. Fig. 3a shows the UV-vis spectra of Au– Ag core–shell NPs solution with a maximum peak at the 405, 411, 416, and 420 nm, respectively. The diameters of Au NPs are 13.1, 18.7, 23.4, and 29.5 nm, respectively. The wavelength of maximum absorbance increased with the increase in the sizes of Au NPs, as shown in Fig. 3b. The inset photos are depicted
Fig. 1 Schematic diagram for preparation of sensing system for
adenosine.
Fig. 2 (a) UV-vis spectra of Au NPs solutions synthesized in different
concentrations of citrate. (b) The wavelength of Au NPs maximum absorbance as a function of the concentration of citrate. The inset
image is the photo of (a). TEM images (scale bar¼ 10 nm) of an Au NP
grown with (c) 15.5 mM, (d) 19.4 mM, (e) 25.9 mM, and (f) 38.8 mM citrate solution, respectively.
Fig. 3 (a) UV-vis spectra of Au–Ag core–shell NPs solutions
synthe-sized on the different sizes of Au NPs. (b) The wavelength of Au–Ag
core–shell NPs maximum absorbance as a function of the sizes of Au
NPs. The inset image is the photo of (a). TEM images (scale bar¼
10 nm) of an Au–Ag core–shell NP synthesized on the different sizes of
Au NP: (c) 13.1 nm, (d) 18.7 nm, (e) 23.4 nm, and (f) 29.5 nm.
from le to right with the color change from yellow to yellow brown. The red shi is also ascribed to the quantum conne-ment effect arising from the increased sizes of the Au–Ag core– shell NPs. When the sizes of Au NPs solution were higher than 18.7 nm, the shoulder band of Au–Ag core–shell NPs solution became relatively evidence. The shoulder in the SPR band can be observed at around 490 nm, which seems to increase in relative intensity as compared with the sizes of Au NPs. The phenomenon is caused by the LSPR of the large Au core.
Fig. 3c–f show TEM images of an Au–Ag core–shell NP with spherical geometry and grown for the diameters of Au NPs at 13.1, 18.7, 23.4, and 29.5 nm, respectively. The TEM images clearly show that the NP with a core–shell structure and the diameters of Au–Ag core–shell NP increase from 24.8, 32.9, 43.5, and 52.9 nm, respectively. The morphology and distribution of different sizes of Au–Ag core–shell NPs can be clearly obtained by low magnication of TEM images (Fig. S3†). The EDS spec-trum (Fig. S4a†) shows that the only element of Ag is present in the side of core–shell NP. In addition, the EDS spectrum (Fig. S4b†) shows that the elements of Au and Ag are present in the center of the core–shell NP. These results conrm that the constituents are the Au core and the Ag shell. The correlation between the varied sizes of Au NPs and the seeding growth approach can be exploited to control the sizes of Au–Ag core– shell NPs.
3.3 Evaluation of self-assembly Au NPs and Au–Ag core–shell NPs
The self-assembly method, which is an essential part of nano-technology, is one of the best non-physical deposition tech-niques for building 2D or 3D nanostructures.29,30 Previous
studies have employed the self-assembly technique utilizing the chemical interaction between NPs and the substrate or struc-ture surface. For instance, organic molecules containing thiol (–SH) or amine (–NH2) can be adsorbed onto the gold or silver
surface to form a well-organized self-assembly monolayer.31In
our previous works, the fabrication of monolayer Au NPs was carried out using immersion or the spin coating method on a 3-aminopropyltrimethoxysilane (APTMS)-modied silicon substrate.32Fig. 4a illustrates the procedure used to fabricate Au
NPs or Au–Ag core–shell NPs onto the silicon substrate. The fabrication processes can be briey divided into two steps: (1) APTMS-functionalization of the silicon substrate and (2) self-assembly of Au NPs or Au–Ag core–shell NPs on APTMS. The rst step deposits a large-scale monolayer APTMS on the silicon substrate with a thin native oxide layer. The second step deposits the Au NPs or Au–Ag core–shell NPs onto the amino-terminated bonding using an immersion method. Aer immersing the substrate in a solution of Au NPs or Au–Ag core– shell NPs for the desired time duration, the substrate is immediately rinsed with de-ionized water and dried under a N2
purge.
Fig. 4b–e show top-view SEM images of the different sizes of Au NPs self-assembled on the Si substrate. The average diam-eters of Au NPs are 12.2 1.8, 17.5 2.7, 20.1 3.5, and 26.8 5.1 nm, respectively. The uniform sizes of Au NPs can be
effectively controlled with the appropriate concentration of citrate as well as the growth condition. The densities and self-assembly time of Au NPs are 2356(15), 1164(30), 723(60), and 396(100) per micron2 (min), respectively. In addition, the surface coverage of Au NPs can be calculated from SEM images at 27.5, 24.9, 22.9, and 22.3%, respectively. The average diam-eters and self-assembly conditions for the different sizes of Au NPs are listed in Table S1.† Appropriate self-assembly time leads to the growth of Au NPs with similar surface coverage. The self-assembly time progressively increases with an increase in the sizes of Au NPs. The result is attributed to decrease in the concentration of Au NPs solution.
The different sizes of Au–Ag core–shell NPs can also be self-assembled on the Si substrate, as shown in Fig. 4f–i. The average diameters of Au–Ag core–shell NPs are 23.4 3.3, 30.9 5.5, 38.4 5.6, and 52.1 8.2 nm, respectively. Compared to the average diameters of Au NPs, the average shell thicknesses of Au–Ag core–shell NPs are 5.6, 6.7, 9.2, and 12.7 nm, respectively. The shell thicknesses of Au–Ag core–shell NPs increase with an increase in sizes of Au NPs. This phenomenon is attributed to decrease in the concentration of Au NPs solu-tion. On the other hand, the shell thicknesses of Au–Ag core– shell NPs can also be controlled by the different sizes of Au NPs. The densities and self-assembly time of Au–Ag core–shell NPs are 641(45), 336(75), 207(120), and 105(180) per micron2(min), respectively. The surface coverage of Au–Ag core–shell NPs can be calculated from SEM images at 27.6, 25.2, 24.0, and 22.4%, respectively. The average diameters and self-assembly condi-tions for the different core sizes of Au–Ag core–shell NPs are listed in Table S2.† In order to achieve the similar surface coverage of NPs, Au–Ag core–shell NPs require a longer self-assembly time. When the self-self-assembly time was longer than 180 min, the Au–Ag core–shell NPs appeared the aggregation and precipitation. This phenomenon is attributed to the occurrence of aggregation during the long self-assembly time of NPs. These results conrm that the self-assembly time plays a crucial role in determining the different sizes of Au NPs and Au– Ag core–shell NPs with similar surface coverage on the silicon
Fig. 4 (a) Schematic diagram for self-assembly NPs. The SEM images
of the different sizes of Au NPs were deposited on the silicon substrate:
(b) 13.1, (c) 18.7, (d) 23.4, and (e) 29.5 nm. The SEM images of the
different sizes of Au–Ag core–shell NPs were deposited on the silicon
substrate: (f) 24.8, (g) 32.9, (h) 43.5, and (i) 52.9 nm.
substrate. The different sizes of Au NPs and Au–Ag core–shell NPs shall be benecial in fabricating SERS substrates.
3.4 SERS-based detection of rhodamine B
SERS effect is very sensitive to the roughness of metal surface.1,7
Herein, the different sizes of Au NPs and Au–Ag core–shell NPs were used as substrate to examine the SERS effect. Rhodamine B was chosen as the probe molecule owing to its well-established vibrational features.33,34Fig. 5a–d show the SERS spectra of the
rhodamine B solution (1 106M) dispersed on the different sizes of Au NPs. The strong and medium-strong Raman bands at 1198, 1279, 1357, 1509, and 1646 cm1are observed on the nanostructure surface. The strong peaks at 1279, 1357, 1509, and 1646 cm1are assigned to the stretching modes of aromatic C–C. The band at 1198 cm1 is assigned to aromatic C–H
bending.33,34An obvious trend shows that the intensity of the
Raman signals at 1646 cm1(Fig. S5†) increase gradually with an increase in the sizes of Au NPs, and the maximum SERS signals are at the sizes of 29.5 nm. The SERS spectra of the rhodamine B solution (1 106 M) are dispersed on the different sizes of Au–Ag core–shell NPs, as shown in Fig. 5e–h. For the different sizes of Au–Ag core–shell NPs, the intensity of the Raman signal at 1646 cm1also reveals similar character-istics. The SERS signals at 1646 cm1 (Fig. S5†) are indeed found to increase with an increase in the sizes of Au–Ag core– shell NPs. Compared to the SERS signals at 1646 cm1, the thin Ag shell can provide for more than twice the SERS enhancement in the different sizes of Au NPs. Herein, the substrates are deposited Au NPs or Au–Ag core–shell NPs with similar surface coverage (Fig. 4). It appears that the sizes and shell thicknesses of Au–Ag core–shell NPs are the main factors affecting SERS enhancement activity at the same surface coverage. The appropriate sizes of Au–Ag core–shell NPs were optimized to yield the greatest SERS effect in the rhodamine B molecule. We believe that not only the high SERS activity of this substrate contributes to its high detection sensitivity, but also to the
sorption effect of the large sizes of Au–Ag core–shell NPs, which helps to improve the detection sensitivity.
3.5 SERS-based detection of adenosine
The present study uses SERS based on a structure-switching aptamer for the detection of adenosine (Fig. 1). First, a DNA duplex is formed through the hybridization between an expanding anti-adenosine aptamer (DNA (1)) and a TMR-labelled DNA (DNA (2)). As the DNA (1)/adenosine complex is more stable than the DNA (1)/DNA (2) duplex, the DNA (1) prefer to form the DNA (1)/adenosine complex rather than the DNA (1)/ DNA (2) duplex and trigger the release of DNA (2) from DNA (1)/ DNA (2) duplex. Second, the silicon substrate with self-assem-bled the varied sizes of Au NPs or Au–Ag core–shell NPs is prepared as SERS substrate and modied with thiolated capture DNA (DNA (3)). Finally, the released DNA (2) is easily hybridized with DNA (3) on the SERS substrate, resulting in an increase in SERS signals.
Fig. 6a–d depict the SERS spectra obtained by using the aptameric sensor on the silicon substrate with self-assembled the different sizes of Au–Ag core–shell NPs at 1 104 M
adenosine. SERS signals were observed withve typical Raman bands appearing in the region from 1100 to 1700 cm1. The peaks of these Raman bands are located at 1215, 1351, 1510, 1533, and 1648. The strong peaks at 1351, 1510, 1533, and 1648 cm1are assigned to the stretching modes of aromatic C–C. The band at 1215 cm1 is assigned to aromatic C–H
bending.16Although the overall SERS signals of TMR show a
similar increase in the response to the target, the band at 1648 cm1was chosen as the marker band for TMR due to its ideal peak prole. The Raman intensity at 1648 cm1(Fig. S6†)
obtained from the different sizes of Au–Ag core–shell NPs. It is clearly observed that the signals increase with an increase in the sizes of the Au–Ag core–shell NPs. The largest Au–Ag core–shell NPs have the maximum SERS signals. The SERS enhancement factor (EF) of the largest Au–Ag core–shell NPs is about 5.4 times
Fig. 5 SERS spectra of rhodamine B (1 106M) absorbed on the
silicon substrate with self-assembled the different sizes of Au NPs and
Au–Ag core–shell NPs. Au NPs: (a) 13.1, (b) 18.7, (c) 23.4, and (d) 29.5
nm. Au–Ag core–shell NPs: (e) 24.8, (f) 32.9, (g) 43.5, and (h) 52.9 nm.
Fig. 6 SERS spectra of the aptameric sensor on the silicon substrate
with self-assembled the different sizes of Au–Ag core–shell NPs at 1
104M adenosine: (a) 24.8, (b) 32.9 (c) 43.5, and (d) 52.9 nm.
compared to the smallest. This indicates that the Au–Ag core– shell NPs as SERS substrates provided strong Raman signals of TMR.
The SERS spectra (Fig. S7†) obtained by using the aptameric sensor on the silicon substrate with the different sizes of Au NPs at 1 104M adenosine. The SERS spectra reveal no Raman peak at 1648 cm1. The Au NPs detection schemes were limited by the complex sample preparation and relatively low enhancement factors compared Ag NPs.35Results indicate that
the Ag shell of Au–Ag core–shell NPs plays a crucial role in aptameric sensor. Fig. 7a–e depict the SERS spectra obtained by using the aptameric sensor on the silicon substrate with self-assembled the largest sizes of Au–Ag core–shell NPs at the different concentration of adenosine. For the different concentration of adenosine, the intensity of the Raman signal at 1648 cm1reveals similar characteristics. The SERS signals at 1648 cm1 (Fig. S8†) are indeed found to increase with an increase in the concentration of adenosine. The peak remains clearly observable in the adenosine solution with concentra-tions as low as 0.1 nM. This indicates that the Au–Ag core–shell NPs as SERS substrates provided strong Raman signals of TMR. The fabrication of DNA-based adenosine sensors is facile, of high reproducibility, and a low detection limit (0.1 nM), which shall be advantageous in applications for other aptamer sensing systems.
4.
Conclusions
In summary, the Au NPs with the different sizes can be synthesized by adjusting the concentration of citrate in an aqueous solution. The different sizes of Au NPs can also be used to grow Au–Ag core–shell NPs by the seeding growth method. The appropriate self-assembly time can be used to control the densities of Au and Au–Ag core–shell NPs with similar surface
coverage on an APTMS-modied silicon substrate. The largest Au–Ag core–shell NPs are benecial in yielding the greatest SERS effect of the rhodamine B molecule on the similar surface coverage. The optimizing substrate also exhibits the best enhancement for the detection of adenosine by a structures-switch aptamer. This aptameric SERS sensor is simple and can detect adenosine as low as 0.1 nM. The SERS-based sensing system continues to have great potential for multiplex and sensitive determination in biochemical and biomedical studies.
Acknowledgements
This study was supportednancially by the National Science Council, Taiwan (NSC 102-2218-E-035-009).
References
1 B. Sharma, R. R. Frontiera, A.-I. Henry, E. Ringe and R. P. Van Duyne, Mater. Today, 2012, 15, 16–25.
2 J. P. Camden, J. A. Dieringer, J. Zhao and R. P. Van Duyne, Acc. Chem. Res., 2008, 41, 1653–1661.
3 V. M. Shalaev and A. K. Sarychev, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 57, 13265–13288.
4 M. Kahl and E. Voges, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 61, 14078–14088.
5 A. D. McFarland, M. A. Young, J. A. Dieringer and R. P. Van Duyne, J. Phys. Chem. B, 2005, 109, 11279–11285.
6 P. Etchegoin, L. F. Cohen, H. Hartigan, R. J. C. Brown, M. J. T. Milton and J. C. Gallop, J. Chem. Phys., 2003, 119, 5281–5289.
7 M. Rycenga, C. M. Cobley, J. Zeng, W. Li, C. H. Moran, Q. Zhang, D. Qin and Y. Xia, Chem. Rev., 2011, 111, 3669– 3712.
8 I. Yoon, T. Kang, W. Choi, J. Kim, Y. Yoo, S.-W. Joo, Q. H. Park, H. Ihee and B. Kim, J. Am. Chem. Soc., 2008, 131, 758–762.
9 J. Ni, R. J. Lipert, G. B. Dawson and M. D. Porter, Anal. Chem., 1999, 71, 4903–4908.
10 L. Fabris, M. Dante, G. Braun, S. J. Lee, N. O. Reich, M. Moskovits, T.-Q. Nguyen and G. C. Bazan, J. Am. Chem. Soc., 2007, 129, 6086–6087.
11 Y. Wang and J. Irudayaraj, Chem. Commun., 2011, 47, 4394– 4396.
12 J.-W. Chen, X.-P. Liu, K.-J. Feng, Y. Liang, J.-H. Jiang, G.-L. Shen and R.-Q. Yu, Biosens. Bioelectron., 2008, 24, 66–71.
13 M. You, Y. Chen, L. Peng, D. Han, B. Yin, B. Ye and W. Tan, Chem. Sci., 2011, 2, 1003–1010.
14 R. Jenison, S. Gill, A. Pardi and B. Polisky, Science, 1994, 263, 1425–1429.
15 Y. Xiang, X. Qian, B. Jiang, Y. Chai and R. Yuan, Chem. Commun., 2011, 47, 4733–4735.
16 J. Chen, J. Jiang, X. Gao, G. Liu, G. Shen and R. Yu, Chem.– Euro. J., 2008, 14, 8374–8382.
17 G. Wang, Y. Wang, L. Chen and J. Choo, Biosens. Bioelectron., 2010, 25, 1859–1868.
Fig. 7 SERS spectra of the aptameric sensor on the silicon substrate
with self-assembled the largest sizes of Au–Ag core–shell NPs at the
different concentrations of adenosine: (a) 0.1 nM, (b) 1 nM (c) 10 nM (d)
100 nM, and (e) 1mM.
18 M. Fan, G. F. S. Andrade and A. G. Brolo, Anal. Chim. Acta, 2011, 693, 7–25.
19 L. Chen, L. Luo, Z. Chen, M. Zhang, J. A. Zapien, C. S. Lee and S. T. Lee, J. Phys. Chem. C, 2009, 114, 93–100.
20 M. E. Stewart, C. R. Anderton, L. B. Thompson, J. Maria, S. K. Gray, J. A. Rogers and R. G. Nuzzo, Chem. Rev., 2008, 108, 494–521.
21 H. Ko, S. Singamaneni and V. V. Tsukruk, Small, 2008, 4, 1576–1599.
22 J.-H. Liu, A.-Q. Wang, Y.-S. Chi, H.-P. Lin and C.-Y. Mou, J. Phys. Chem. B, 2004, 109, 40–43.
23 P. Mulvaney, Langmuir, 1996, 12, 788–800.
24 M. Moskovits, I. Srnova-Sloufova and B. Vlckova, J. Chem. Phys., 2002, 116, 10435–10446.
25 R. G. Freeman, M. B. Hommer, K. C. Grabar, M. A. Jackson and M. J. Natan, J. Phys. Chem., 1996, 100, 718–724. 26 J. Turkevich, P. C. Stevenson and J. Hillier, Discuss. Faraday
Soc., 1951, 11, 55–75.
27 R. Levicky, T. M. Herne, M. J. Tarlov and S. K. Satija, J. Am. Chem. Soc., 1998, 120, 9787–9792.
28 Z.-j. Jiang and C.-y. Liu, J. Phys. Chem. B, 2003, 107, 12411– 12415.
29 F.-K. Liu, P.-W. Huang, Y.-C. Chang, F.-H. Ko and T.-C. Chu, Langmuir, 2005, 21, 2519–2525.
30 V. Chechik, R. M. Crooks and C. J. M. Stirling, Adv. Mater., 2000, 12, 1161–1171.
31 F.-K. Liu, Y.-C. Chang, F.-H. Ko, T.-C. Chu and B.-T. Dai, Microelectron. Eng., 2003, 67–68, 702–709.
32 T.-H. Chang, Y.-C. Chang, F.-K. Liu and T.-C. Chu, Appl. Surf. Sci., 2010, 256, 7339–7343.
33 M. Voln´y, A. Sengupta, C. B. Wilson, B. D. Swanson, E. J. Davis and F. Tureˇcek, Anal. Chem., 2007, 79, 4543–4551. 34 T. Z. Melvin, L. Kirsty, W. Ngo-Yin, M. Hong and K. Y. J. Alex,
Nanotechnology, 2007, 18, 455301.
35 R. J. Stokes, A. Macaskill, P. J. Lundahl, W. E. Smith, K. Faulds and D. Graham, Small, 2007, 3, 1593–1601.
