Subscriber access provided by UNIV OF CAMBRIDGE
Article
Plasmonic Au-Ag Janus NPs Engineered Ratiometric SERS Aptasensor for OTA Detection Fangjie Zheng, Wei Ke, Lixia Shi, Han Liu, and Yuan Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02469 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
Plasmonic Au-Ag Janus NPs Engineered Ratiometric SERS
2
Aptasensor for OTA Detection
3
Fangjie Zheng, Wei Ke, Lixia Shi, Han Liu, and Yuan Zhao*
4
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical
5
and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, China.
6
Corresponding author. E-mail:
[email protected] 7 8
ABSTRACT: Ochratoxin A (OTA), a toxic mycotoxin, poses severe risks to environment and
9
human health. Herein, we develop a ratiometric Surface-Enhanced Raman Scattering (SERS)
10
aptasensor based on internal standard (IS) methods for the sensitive and reproducible quantitative
11
detection of OTA. Au-Ag Janus nanoparticles (NPs) are successfully synthesized under the
12
guidance of 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA), which possesses intrinsic
13
Raman signals, thus no additional modification with Raman reporter on NPs is required. In
14
addition, Au-Ag Janus NPs exhibit amplified and stable SERS activity. MXenes nanosheets
15
generate a unique and stable Raman signal, making them an ideal IS for quantitative Raman
16
analysis. In principle, Au-Ag Janus NPs are assembled with MXenes nanosheets depending on
17
hydrogen bond and the chelation interaction between MXenes nanosheets and OTA aptamers. In
18
the presence of OTA, Au-Ag Janus NPs are dissociated from MXenes nanosheets due to the
19
formation of aptamer/OTA complex, leading to the attenuation of Raman signal of Au-Ag Janus
20
NPs, and meanwhile, the signal of MXenes nanosheets remain constant. Quantitatively, upon
21
correction by the IS Raman signals, sensitive and quantitative detection can be achieved with the
22
limit of detection (LOD) of 1.28 pM for OTA. Our results suggest that this ratiometric SERS
23
aptasensor is a powerful tool which shows great promise for applications in complex systems.
24
KEYWORDS: Au-Ag Janus NPs, MXenes nanosheets, SERS, OTA
25 26
1
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Ochratoxin is known as a powerful mycotoxin produced by several species of Aspergillus
2
and Penicillium.1 It is one of the most toxic and hazardous contaminants and occurs widely in
3
variety of species, causing a threat to human health.2-4 In particular, ochratoxin A (OTA), which is
4
a member of the ochratoxin family, has the largest toxicity, the widest distribution and the highest
5
toxic yield.5 OTA exists ubiquitously in a variety of foodstuffs including cereals, coffee beans,
6
beans, grapes and dried fruits, thus ultimately finding its way to the human body through food
7
chain.6 Consequently, OTA has become a major concern in global food safety issues. The
8
European Commission established the maximum allowable concentrations for OTA as low as 5
9
μg/kg from raw cereal grains.7,
10
8
Thus, it is of great urgency for researchers to identify and
quantify OTA to meet food safety requirements.
11
Recently, various analytical methods have been developed to achieve the sensitive detection
12
of OTA, including high-performance liquid chromatography (HPLC), gas chromatography-mass
13
spectrometry (GC-MS), high-performance liquid fluorescence detection (HPLC-FLD) and
14
immunoassay.9 In spite of the numerous advantages that such techniques offer, these conventional
15
detecting methods using HPLC or MS require high cost, sophisticated instrumentation and
16
complicated time-consuming process.10-12 Furthermore, the immunoassay using antibody
17
molecules as a recognition element for OTA detection depends on both special reagents and
18
expertise to run the tests. Thus, a more simple, rapid, sensitive, and low-cost detection method for
19
high-sensitivity detection of OTA is still required.
20
Surface-enhanced Raman scattering (SERS) is a powerful fingerprint spectroscopy which can
21
detect small molecules at a single molecule level without complex sample treatment
22
procedures.13-15 It has been applied in many different fields involving chemical, physical and
23
biological sciences.16, 17 The primary one of the two widely accepted theories for the enhancement
24
effect is electromagnetic mechanism predicting that the local amplification of electromagnetic
25
field is originated from the unique plasmon coupling between neighboring nanoparticles
26
(NPs).18-20 Thus, SERS generally requires noble-metal NPs with strong SPR effect as substrate
27
materials.21 However, the SERS signals derived from single NP are too weak for wide application
28
in ultrasensitive detection.22 As a result, different structures were developed to obtain strong SERS
29
intensity. Janus NPs represent a special category of anisotropic NPs.23 Au-Ag Janus NPs can 2
ACS Paragon Plus Environment
Page 2 of 25
Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
integrate the advantages of both materials within one nanostructure.24 For instance, Au NPs have
2
good stability and Ag NPs have high plasmonic effects.25 The plasmonic coupling between
3
bimetallic NPs results in gigantic EM fields. Generally, organic molecules are frequently used as
4
Raman
5
4-mercaptobenzoic acid (4-MBA), etc. But using these conventional organic molecules modified
6
on the surface of NPs has its problems since they are not steady under complex environmental
7
conditions. Interestingly, 2-mercaptobenzoimidazole-5- carboxylic acid (MBIA) is used to guide
8
the oriented growth of Ag islands to form Au-Ag Janus NPs and also possesses Raman signals,
9
which would be amplified by the strong coupled plasmons in the junctions between Au heads and
10
Ag islands.26-28 No extra Raman molecules were further needed to modify on the surface of NPs.
11
MBIA was embedded in the junction of Au-Ag Janus NPs, and the SERS signals are stable and
12
strong, promoting more sensitive detection.
reporter
such
as
4-aminothiphenol
(4-ATP),
4-nitrothiophenol
(4-NTP),
13
An ideal SERS aptasensor should include the characteristics: strong SPR effect, high
14
stability, high reproducibility and good versatility.29 However, accurate and reproducible
15
quantification for SERS aptasensors is still a great challenge.30 To solve this problem, numerous
16
strategies have been tried including introducing an internal standard (IS) into the SERS system.31
17
Generally, the IS employed in SERS can calibrate the variances caused by instrument factors and
18
measuring conditions.32 MXenes, a large family of emerging two-dimensional (2D) materials,
19
have been aroused much attention and already demonstrated promise in biosensing and other
20
applications.33, 34 The MXenes generally exhibit superior chemical stability, hydrophilic properties
21
and large surface areas. The most common MXenes, Ti3C2Tx, where Tx represents the surface
22
terminations (-OH, -F, -O), have been demonstrated as SERS substrates.35-37 MXenes can interact
23
with a majority of biomolecules via hydrogen bond, coordination bonds, van der Waals force,
24
electrostatic interaction, and so on.38, 39 The large surface area of MXenes nanosheets made them
25
had a particular advantage of loading NPs. Moreover, the MXenes nanosheets exhibit quite
26
distinctive and stable Raman signal, which can be used as IS signal to calibrate deviation in the
27
SERS detection.40
28
In this work, a ratiometric SERS aptasensor based on the IS methods for the sensitive and
29
quantitative detection of OTA is reported. Au-Ag Janus NPs were synthesized through the 3
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 25
1
sequential growth of Ag islands on Au cores. The ligand MBIA worked as a Raman reporter and
2
Au-Ag Janus structures can greatly amplify the SERS signal. Then we combine the Au-Ag Janus
3
NPs with highly stable MXenes nanosheets which serve as an internal standard depending on the
4
hydrogen bond and the chelation interaction between the phosphate groups and Ti ions on the
5
basal plane of MXenes. MXenes nanosheets exhibit a unique and stable Raman signal, which can
6
be ideally used as an IS for quantitative analysis. Combining the inherent advantages of
7
ratiometric strategy and SERS, this IS-apatsensor is able to detect OTA with high sensitivity and
8
good reproducibility, which holds great potential for the practical application for other analytes
9
detection.
10
EXPERIMENTAL SECTION
11
Materials. Chloroauric acid (HAuCl4·4H2O), sodium citrate, silver nitrate (AgNO3),
12
hydroquinone (HQ), hydrofluoric acid (HF), dimethyl sulfoxide (DMSO) and bovine serum
13
albumin (BSA) were all obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Titanium
14
aluminum carbide powder (Ti3AlC2) was obtained from Shanghai Maclean Biochemical
15
Technology Co., Ltd. Ochratoxin A (OTA), aflatoxin B1 (AFB1), and fumonisin B1 (FB1) were
16
obtained from Sigma-Aldrich. 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA) and
17
microcystin-LR (MC-LR) were obtained from J&K Scientific Ltd. Aptamer purified by HPLC was
18
obtained from Shanghai Sangon Biological Engineering Technology & Services Co., Ltd. (Table
19
S1). All reagents were of analytical grade and were used without further purification. All solutions
20
were prepared with deionized water (18.2 MΩ) obtained from a Milli-Q water purifying system.
21
Instruments. The morphology of NPs was characterized via JEM-2100 plus TEM operated at
22
200 kV. The UV-vis absorbance spectra of NPs were recorded by UV/Vis spectrophotometer
23
(Model
24
(RenishawinVia) with WiRE 3.3 software.
TU-1901).
Raman
spectra
were
obtained
with
Micro-Raman
spectrometer
25
Synthesis of Monodisperse Au NPs. The Au NPs used as seeds was synthesized by the
26
citrate-reduction method. In a typical synthesis, 1 ml 1% HAuCl4 in 99 mL of Milli-Q water were
27
heated to boiling. Then, 1 mL 1% sodium citrate was quickly injected into the boiling solution
28
under vigorously stirring. The solution was kept boiling for 15 min and was cooled down to room
29
temperature naturely. 4
ACS Paragon Plus Environment
Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
Synthesis of Au-Ag Janus NPs. To 1 mL as-synthesized citrate-stabilized Au NPs, 20 μL 1
2
mM 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA, in ethanol) was added under vigorous
3
vortex. The mixture was incubated in a 60 0C oven for 2 h. After the solution was cooled down to
4
room temperature, 60 μL 10 mM hydroquinone (HQ) and 1 mM AgNO3 were added in sequence
5
into the above solution under vigorous vortex. The resulting solution was placed undisturbed at
6
room temperature for at least 2 h to allow the complete reduction of AgNO3. When 60 μL, 80 μL,
7
100 μL, 120 μL, 160 μL, 200 μL, 240 μL AgNO3 solutions were respectively used, Au-Ag Janus
8
NPs named as Au-Ag(1) Janus NPs, Au-Ag(2) Janus NPs, Au-Ag(3) Janus NPs, Au-Ag(4) Janus NPs,
9
Au-Ag(5) Janus NPs, Au-Ag(6) Janus NPs and Au-Ag(7) Janus NPs were obtained.
10
Immobilization of Aptamers. An aliquot of 50 μL Au-Ag Janus NPs was mixed with 1 μM
11
aptamer-O (Table S1). The coupling ratio of aptamers to NPs was optimized to 1:1. The mixtures
12
were incubated in 200 μL 1×TBE containing 50 mM NaCl for 12 h, and then were centrifuged to
13
remove the excess aptamers. Aptamers modified Au-Ag Janus NPs were dispersed in 100 μL
14
ultrapure water and stored at 4 °C before use.
15
Preparation of MXenes Nanosheets. In a typical experiment, a total of 7.5 mL of HF was
16
slowly added to 0.5 g Ti3AlC2 powder and kept at 45 0C for 24 h under magnetic stirring. Then the
17
reaction mixture was washed several times with Milli-Q water until the pH was greater than or
18
equal to 6. The product was dried at room temperature to obtain etched MXenes material. For
19
delamination, 0.3 g of MXenes powder was added into 5 mL of DMSO at room temperature, and
20
stirred for 24 h. Then, the solution was centrifuged and washed to remove DMSO. The obtained
21
products were sonicated for 2 h, followed by centrifugation (4000 rpm) for 1 h to obtain
22
supernatant containing MXenes nanosheets colloidal solution.
23
Fabrication of SERS-active Ratiometric Aptasensor. The prepared apt-Au-Ag Janus NPs
24
solution and MXenes nanosheets colloidal solution were mixed in 100 μL of ultrapure water and
25
incubated for 12 h. Au-Ag Janus NPs-MXenes assemblies were fabricated through the hydrogen
26
bond and the chelation interaction between the phosphate groups in the aptamers and Ti ions on
27
the basal plane of MXenes. Then the assemblies were purified through centrifugation (4000 rpm)
28
and dissolved in 100 μL of ultrapure water for following detection. Au-Ag Janus NPs-MXenes
29
assemblies employed as SERS-active aptasensors were mixed with 100 μL different concentration 5
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
of OTA solutions, i.e., 0, 0.01, 0.05, 0.1, 0.5, 1, 5, 10 and 50 nM. The mixture was incubated in 20
2
mM Tris-HCl solution for 1 h at room temperature to ensure complete recognition between
3
aptamers and OTA. Then the solutions were centrifuged to remove the disassembled Au-Ag Janus
4
NPs and were washed by 70% ethanol aqueous solution. SERS spectra of assemblies were
5
determined through the Renishaw inVia Micro-Raman spectrometer with a 785 nm Ar+ ion laser
6
source at room temperature. SERS spectra were further managed through the WiRE 3.3 software.
7
Specificity Evaluation. The selectivity of SERS-active ratiometric aptasensor was studied in
8
the presence of 5 nM other mycotoxin and proteins, including BSA, AFB1, FB1 and MC-LR.
9
After incubated for 1 h, the residual assemblies were separated from the mixture and were washed
10
three times. The residual assemblies were further analyzed by Raman instrument.
11
Real Samples Detection. For the detection of real samples, the accuracy and the feasibility of
12
the aptasensor was investigated by detecting the different concentration of OTA standard solutions
13
spiked in red wine samples, including 0.1, 1.0, 5.0 and 10.0 nM. The pretreatment of the red wine
14
samples was based on official methods of China (GB/T 15038-2006). Dilute red wine samples
15
were added into a flask, and the mixture was heated to distillation gently. Then the distilled
16
solution was collected to use as the sample for real samples detection of OTA.
17
RESULTS AND DISCUSSION
18
Strategy of the IS-aptasensor for OTA Detection. The SERS sensing strategy for the
19
detection of OTA is illustrated in Scheme 1. Citrate-stabilized Au NPs were synthesized and
20
treated with MBIA to achieve nearly complete ligand coverage on their surface. The ligand MBIA
21
has a -SH group and a diametric -COOH group, allowing it to interact strongly both with the Au
22
NPs and the subsequent Ag island. Thus, Ag atoms were deposited on Au NPs to form an Ag
23
island by reducing AgNO3. The as prepared Au-Ag Janus NPs exhibited intrinsic enhanced SERS
24
signal. Under optimized condition, aptamers modified Au-Ag Janus NPs could be connected to
25
MXenes nanosheets through the hydrogen bond and the chelation interaction between the
26
phosphate groups and Ti ions. MXenes nanosheets whose Raman signal was stable were used as
27
an internal standard. In the absence of OTA, the Raman signals from both Au-Ag Janus NPs and
28
MXenes nanosheets could be detected and identified. In the presence of OTA, the OTA aptamer
29
preferred to switch its configuration to combine with the OTA target, inducing the dissociation of 6
ACS Paragon Plus Environment
Page 6 of 25
Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
each Au-Ag Janus NPs from MXenes nanosheets and further decreasing the SERS signal.
2
However, the Raman signal of the internal MXenes nanosheets was not affected by the external
3
factors, realizing the quantitative detection of OTA based on the ratiometric peak intensity of
4
IAu-Ag
5
from environmental factors would be overcome through the ratiometric strategy. There was a
6
linear relationship (negative correlation) between the SERS ratiometric peak intensity and the
7
concentration of OTA.
Janus/IMXenes
(I1278/I730). Moreover, the problem of SERS signal irreproducibility resulting
8
Characterizations of Au-Ag Janus NPs. As-prepared Au NPs showed uniform morphology
9
(32.8 ± 0.2 nm) and good dispersivity as shown in Figure 1a. The citrate-stabilized Au NPs were
10
incubated with ligand MBIA at 60 0C for 2 h and were used as seeds for growing Ag. With the
11
start of Ag reduction by the reductant HQ, the resulting Ag atoms were deposited on the Au seed
12
surface, forming Ag islands. As shown in Figure 1b, small Ag islands with an average diameter of
13
7.1 ± 0.1 nm were deposited on the surfaces of Au NPs. The average length to diameter ratio of
14
Au-Ag(1) Janus NPs was about 5.6 ± 0.1 nm (Figure 2a). When the volume of AgNO3 was
15
increased from 60 μL to 240 μL, Ag islands were getting bigger (Figure 1b-h). The size of the Ag
16
islands increased from 7.1 nm to 58.2 nm (Figure 2a). Other than this, the grown Ag islands were
17
quite segregated from the Au seeds, so that the hybrid NPs almost appeared as two juxtaposed
18
NPs. The average length to diameter ratio of Au-Ag Janus NPs first became smaller and then
19
gradually became larger (Figure 2a). Au-Ag(4) Janus NPs had the smallest length to diameter ratio,
20
which is 2.08. When the volume of AgNO3 reached to 240 μL, the grown Ag islands tended to
21
become irregular (Figure 1h). Enlarged images revealed the process of change of Ag island which
22
deposited on the Au seed surface.
23
The optical properties of Au-Ag Janus NPs were recorded by UV-vis absorption spectra (Figure
24
2b). 33 nm Au NPs with a light red color showed a unique absorption peak at 528 nm. Ag NPs
25
showed a yellow color with an absorption peak at about 400 nm. When small Ag islands began to
26
deposit on the surfaces of Au NPs, the absorption peak of the formed Au-Ag(1) Janus NPs slightly
27
blue-shifted to 520 nm. It showed a light red color similar to Au NPs. For Au-Ag(2) Janus NPs, a
28
new absorption peak for Ag islands appeared at 394 nm. Simultaneously, the absorption peak at
29
526 nm corresponding to Au NPs broadened in the extinction spectra. The color of Au-Ag(2) Janus 7
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
NPs solution changed to pink purple. When 100 μL AgNO3 was added during the reaction, the
2
formed Au-Ag(3) Janus NPs whose color was blue purple showed three bands: Ag transverse
3
plasmon absorption at around 400 nm, weak Au transverse absorption at around 525 nm, and
4
Au-Ag longitudinal absorption at around 630 nm. When the volume of AgNO3 increased from 100
5
μL to 240 μL, the Ag transverse plasmon absorption peak gradually red-shifted and broadened and
6
Au transverse absorption peak gradually disappeared. Correspondingly, the color of NPs changed
7
from blue to green and then turned yellow.
8
Enhanced Raman effect of Au-Ag Janus NPs with different sizes of Ag islands was observed
9
when MBIA was used as the ligand. Figure 2c showed the SERS spectra of Au-Ag Janus NPs
10
under the same final concentration. MBIA worked as a Raman reporter. The major characteristic
11
peaks in the Raman spectra are located at 463 cm-1, 818 cm-1, 1227 cm-1, 1278 cm-1 and 1408
12
cm-1. Among these characteristic peaks, the intensity of peaks at 1278 cm-1 and 1408 cm-1 are
13
stronger and they have the same tendency to increase or decrease. MBIA modified Au NPs
14
showed relative low strength SERS enhancement. In comparison to Au NPs, a slight enhancement
15
of SERS intensity was observed for Au-Ag(1) Janus NPs. When the Ag islands grew bigger, the
16
Raman intensity gradually increased, then slightly decreased. We found that the Raman signal of
17
MBIA modified Ag NPs was stronger than that of Au-Ag(1) Janus NPs and Au-Ag(2) Janus NPs.
18
The reason may be that Ag NPs had a broader electromagnetic region of activity and the best
19
plasmonic enhancement and the Ag islands of Au-Ag(1) Janus NPs and Au-Ag(2) Janus NPs were
20
too small to couple with Au NPs to generate electromagnetic fields. Therefore, Au-Ag Janus NPs
21
with larger Ag islands exhibited stronger SERS signals due to the electromagnetic field
22
enhancement. Considering the positions and the corresponding Raman intensity of the SERS
23
spectrum, we chose the Raman peak at 1278 cm-1 as the identification position for the quantitative
24
analysis. In order to clearly observe the relationship between the SERS intensity and the sizes of
25
Ag islands, we plotted the SERS intensity change of the characteristic peak at 1278 cm-1, as shown
26
in Figure 2d. It is observed that with the Ag islands gradually became larger, the SERS signal first
27
increased and then weakened slightly. SERS signal of Au-Ag(5) Janus NPs was more than 20 fold
28
of magnitude stronger than that of MBIA modified Au cores. The largest intensity was achieved
29
on Au-Ag(5) Janus NPs. Due to the electromagnetic field enhancement between the Au cores and 8
ACS Paragon Plus Environment
Page 8 of 25
Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
Ag islands, a largely amplified SERS intensity was observed. Although Au-Ag(6) Janus NPs and
2
Au-Ag(7) Janus NPs also have stronger SERS signals, irregularity of the Ag islands may result in
3
uneven signals. Considering the good dispersion and structural uniformity, we chose Au-Ag(5)
4
Janus NPs to be used as SERS probes for the OTA detection.
5
Synthesis and Characterization of MXenes nanosheets. The MXenes nanosheets were
6
prepared from Ti3AlC2 by selectively etching the metallic Al-Ti bonds with HF. The structure and
7
morphology of the as-prepared MXenes nanosheets were characterized by TEM. As shown in
8
Figure 3a, monodisperse morphology could be seen, confirming the successful exfoliation of few
9
or single layer MXenes nanosheets. Besides, the well isolated nanosheets were nearly transparent.
10
The UV-vis absorption spectrum of MXene nanosheets in water (Figure 3b) had a broad
11
absorption around 800 nm and strong absorption around 340 nm, which could be attributed to their
12
interband transitions. Raman spectrum by MXenes nanosheets acquired in twenty continuous
13
measurements at the same spot was shown in Figure 3c. Two evident bands could be observed at
14
213 cm-1 and 730 cm-1, corresponding to the Ti-C and C-C vibrations (A1g symmetry) of the
15
oxygen-terminated Ti3C2O2, respectively. Another two peaks could be observed at approximately
16
365 cm-1 and 625 cm-1, where the former attributed to the O atoms Eg vibrations and the latter
17
came mostly from Eg vibrations of the C atoms in the OH-terminated MXenes nanosheets. The
18
evident band at 730 cm-1 was strong enough for use as an ideal internal standard for SERS
19
quantitative analysis. Furthermore, the twenty SERS spectra demonstrated superior consistency,
20
which indicated the excellent reproducibility and surface properties of the MXenes nanosheets.
21
This excellent reproducibility allowed MXenes nanosheets to act as an internal standard.
22
Stability of Au-Ag Janus NPs and MXenes nanosheets for Raman Analysis. Stable Raman
23
probes are critical for the sensitive and accurate detection. Generally, modifying of Raman-active
24
molecules such as 4-mercaptobenzoic acid (MBA) or 4-aminothiophenol (ATP) on a metal layer
25
could gave rise to greatly enhanced SERS signals. However, using these conventional organic
26
molecules has its problems since they are not steady in harsh environments. Herein, MBIA is used
27
to guide the oriented growth of Ag islands and also possesses Raman signals, thus there is no need
28
to modify extra Raman-active molecules on the surface of Au-Ag Janus NPs. We investigated the
29
stability of Au-Ag Janus NPs as Raman probes. As plotted in Figure 4a and 4c, there were no 9
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
obvious changes of Raman intensities at 1278 cm-1 before and after incubation under different pH
2
conditions (4, 5, 6, 7, 8), temperatures (20, 30, 40, 50 0C) and storage times (0, 30, 60, 90 days).
3
The results indicated that this intrinsic Raman signals of Au-Ag Janus NPs are stable and strong
4
enough for sensitive detection. Apart from this, an ideal internal standard should be stable,
5
homogeneous, and reproducible, which still remained challenging in SERS analysis. MXenes
6
nanosheets were selected because of its stable SERS signals. As illustrated in Figure 4b and 4d,
7
after MXenes nanosheets were incubated under different conditions, the Raman intensity was in
8
concordance, indicating that MXenes nanosheets exhibited stable ratiometric signals. Above all,
9
these results indicated that the superior stability of Au-Ag Janus NPs and MXenes nanosheets by
10
its resistance of external influence factor, facilitating wide spread applications in a complex
11
environment.
12
Characterization, Reproducibility and Stability of Au-Ag Janus NPs-MXenes Assemblies.
13
Au-Ag Janus NPs-MXenes assemblies were prepared depending on the hydrogen bond and the
14
chelation interaction. The image inset in Figure 5a showed that Au-Ag(5) Janus NPs were regularly
15
assembled on the surface of MXenes nanosheets, indicating the feasibility of the assembly
16
process. The UV-vis absorption spectrum of the samples obtained by the assembly process were
17
depicted in Figure 5a. Three characteristic bands at 400 nm, 525 nm and 630 nm which were
18
essentially identical to that of Au-Ag(5) Janus NPs were still observed for assemblies. In addition,
19
the assembly dispersion possessed absorption in the visible-near infrared region, which could be
20
attributed to the presence of MXenes nanosheets. The results demonstrated the completion of the
21
assembly process. Corresponding Raman spectra of the assemblies was obtained. As illustrated in
22
Figure 5b, the typical spectrum of Au-Ag(5) Janus NPs (1278 cm-1) and MXenes nanosheets (730
23
cm-1) could be clearly distinguished and the two characteristic peaks do not overlap, indicating the
24
feasibility of simultaneous usage of these two signals for establishing the IS-aptasensor.
25
To further quantitatively examine the reliability and reproducibility of this ratiometric design,
26
we next investigated the SERS performance of the IS-apatsensor. Figure 5c showed the Raman
27
spectra up to twenty rounds with the original spectrum. These twenty consecutive measurements
28
gave rise to nearly identical results and the RSD values of the ratiometric Raman signals were
29
calculated to be 3.15% (Figure 5d), confirming excellent reproducibility of the SERS aptasensor 10
ACS Paragon Plus Environment
Page 10 of 25
Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
for quantification. The stability of the IS-aptasensor was also investigated. As shown in Figure
2
S1a-d, after incubated in different storage times (0, 3, 7, 14 days), temperatures (20, 30, 40, 50
3
0C),
4
signals remained consistent, which meaned that the as-prepared aptasensor were quite stable and
5
robust. Both excellent reproducibility and stability illustrated the feasibility of the IS-aptasensor
6
which could be used for sensitive OTA detection.
salt concentrations (0, 5, 10, 15, 20 mM), and pH values (6, 6.5, 7, 7.5, 8), the ratiometric
7
Sensitivity of OTA Detection and Selectivity Evaluation. To realize the benefits of the
8
ratiometric design, this IS-aptasensor was applied to the detection of OTA in an aqueous solution.
9
The Raman peak at 1278 cm-1 which showed the strongest intensity was chosen as the
10
identification position for the quantitative analysis of OTA, while the peak of MXenes nanosheets
11
at 730 cm-1 was regarded as the internal standard. As shown in Figure 6a, the Raman intensity of
12
Au-Ag(5) Janus NPs at 1278 cm-1 decreased along with the increase of OTA concentration (0.01 to
13
50 nM), displaying a concentration-dependent behavior. However, the Raman signals at 730 cm-1
14
which attributed to MXenes nanosheets, remained unchanged. Therefore, the ratiometric peak
15
intensity I1278/I730 decreased, achieving ratiometric SERS detection of OTA. The corresponding
16
trend of Raman intensity with OTA concentration was further quantitatively evaluated (Figure
17
6b). Figure S2 presents the relationship between the concentration of OTA and the Raman
18
intensity. Notably, without correction by the SERS signals at 730 cm-1 of MXenes nanosheets, a
19
poor linear relationship between the logarithmic concentration of OTA and the corresponding
20
SERS intensities at 1278 cm-1 was observed (red points in Figure 6b, Figure S2). On the contrary,
21
a good linearity (R2=0.998) between the logarithmic concentration of OTA and the ratiometric
22
Raman signals was achieved, demonstrating that an internal standard could improve the accuracy
23
of quantification (Figure 6c). Remarkably, the LOD was calculated to be 1.28 pM by setting the
24
signal-to-noise ratio of 3:1. The developed sensor also possessed good selectivity. As summarized
25
in Figure S3, a series of Raman spectra were collected after being incubated with 5 nM
26
nonspecific molecules, involving OTA, BSA, AFB1, FB1, MC-LR and the control groups without
27
any targets. One can see from the results that none of these interfering mycotoxin and proteins
28
caused an obvious change of ratiometric peak intensity as OTA did (Figure 6d). This phenomenon
29
was easily understood by the favorite affinity of aptamers and OTA. These results demonstrate 11
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
that the IS-aptasensor had an excellent selectivity towards OTA detection.
2
Red Wine Samples Detections. To validate the practical applicability of the proposed SERS
3
IS-aptasensor for the detection of OTA, the analysis of spiked samples was considered. We
4
choosed red wine as a sample model to use in the real sample detection with adding different
5
concentrations of OTA (0.10, 1.00, 5.00 and 10.00 nM). The Raman spectra and statistical
6
analysis of the ratiometric peak intensity were obtained in Figure 7a-b. The recovery rates were
7
calculated in the range of 93.31% ± 2.65% to 97.44% ± 0.87%. The RSD of this method was
8
within 2.65% (Table S2). Consequently, the potential application of the ratiometric SERS strategy
9
in real environmental samples was promising.
10
CONCLUSIONS
11
In summary, we have successfully developed a ratiometric SERS aptasensor for OTA detection
12
based on Au-Ag Janus NPs-MXenes assemblies with high sensitivity and excellent signal
13
reproducibility. The prepared Au-Ag Janus NPs exhibits amplified and stable SERS signal
14
originating from the ligand MBIA which participates in the synthesis reaction. MXenes
15
nanosheets used as an internal standard demonstrates stable Raman vibration bands. Coexisting
16
interferences in the detection system, which could affect the accuracy of Raman analysis, were
17
effectively eliminated with utilizing the proposed SERS IS-aptasensor. More importantly, the
18
ratiometric SERS sensing approach has enhanced the sensitivity and reliability of OTA detection
19
in complex systems. Above all, we envision that this novel ratiometric SERS strategy can be
20
extended to the sensitive, reproducible and quantitative determination of other target analytes in
21
real systems.
22
ASSOCIATED CONTENT
23
Supporting Information
24
The Supporting Information is available free of charge on the e on the ACS Publications website
25
at DOI: 10.1021/acs.anal-chem.xxxx.
26
Stability of Au-Ag Janus NPs-MXenes assemblies, linear relationship, SERS spectra of the
27
selectivity evaluation, detail sequences of aptamers of OTA, and real samples detection of OTA
28
(PDF).
29
AUTHOR INFORMATION 12
ACS Paragon Plus Environment
Page 12 of 25
Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
Corresponding Author
2
E-mail:
[email protected] 3
Notes
4
The authors declare no competing financial interest.
5
ACKNOWLEDGMENTS
6
This work is financially supported by the National Key Research and Development Program of
7
China (2017YFC1601706), Natural Science Foundation of Jiangsu Province (BK20171136),
8
National First-class Discipline Program of Food Science and Technology (JUFSTR20180302),
9
and the 111 Project (B13025).
10 11
REFERENCES
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
1.
Rivas, L.; Mayorga-Martinez, C. C.; Quesada-Gonzalez, D.; Zamora-Galvez, A.; de la
Escosura-Muniz, A.; Merkoci, A., Label-free impedimetric aptasensor for ochratoxin-A detection using iridium oxide nanoparticles. Analytical chemistry 2015, 87 (10), 5167-72. 2.
Viter, R.; Savchuk, M.; Iatsunskyi, I.; Pietralik, Z.; Starodub, N.; Shpyrka, N.; Ramanaviciene,
A.; Ramanavicius, A., Analytical, thermodynamical and kinetic characteristics of photoluminescence immunosensor for the determination of Ochratoxin A. Biosensors & bioelectronics 2018, 99, 237-243. 3.
Ni, J.; Yang, W.; Wang, Q.; Luo, F.; Guo, L.; Qiu, B.; Lin, Z.; Yang, H., Homogeneous and
label-free electrochemiluminescence aptasensor based on the difference of electrostatic interaction and exonuclease-assisted target recycling amplification. Biosensors & bioelectronics 2018, 105, 182-187. 4.
Hu, S.; Ouyang, W.; Guo, L.; Lin, Z.; Jiang, X.; Qiu, B.; Chen, G., Facile synthesis of Fe 3 O 4
/g-C 3 N 4 /HKUST-1 composites as a novel biosensor platform for ochratoxin A. Biosensors and Bioelectronics 2017, 92, 718-723. 5.
Duan, H.; Huang, X.; Shao, Y.; Zheng, L.; Guo, L.; Xiong, Y., Size-Dependent
Immunochromatographic Assay with Quantum Dot Nanobeads for Sensitive and Quantitative Detection of Ochratoxin A in Corn. Analytical chemistry 2017, 89 (13), 7062-7068. 6.
Feng, J.; Li, Y.; Gao, Z.; Lv, H.; Zhang, X.; Fan, D.; Wei, Q., Visible-light driven label-free
photoelectrochemical immunosensor based on TiO2/S-BiVO4@Ag2S nanocomposites for sensitive detection OTA. Biosensors & bioelectronics 2018, 99, 14-20. 7.
Wang, Y.; Ning, G.; Wu, Y.; Wu, S.; Zeng, B.; Liu, G.; He, X.; Wang, K., Facile combination of
beta-cyclodextrin host-guest recognition with exonuclease-assistant signal amplification for sensitive electrochemical assay of ochratoxin A. Biosensors & bioelectronics 2019, 124-125, 82-88. 8.
Liu, R.; Huang, Y.; Ma, Y.; Jia, S.; Gao, M.; Li, J.; Zhang, H.; Xu, D.; Wu, M.; Chen, Y.; Zhu, Z.;
Yang, C., Design and synthesis of target-responsive aptamer-cross-linked hydrogel for visual quantitative detection of ochratoxin A. ACS applied materials & interfaces 2015, 7 (12), 6982-90. 9.
Kong, D.; Liu, L.; Song, S.; Suryoprabowo, S.; Li, A.; Kuang, H.; Wang, L.; Xu, C., A gold
nanoparticle-based semi-quantitative and quantitative ultrasensitive paper sensor for the detection of twenty mycotoxins. Nanoscale 2016, 8 (9), 5245-53. 13
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
10. Song, S.; Liu, N.; Zhao, Z.; Njumbe Ediage, E.; Wu, S.; Sun, C.; De Saeger, S.; Wu, A., Multiplex lateral flow immunoassay for mycotoxin determination. Analytical chemistry 2014, 86 (10), 4995-5001. 11. Wang, S.; Zhang, Y.; Pang, G.; Zhang, Y.; Guo, S., Tuning the Aggregation/Disaggregation Behavior of Graphene Quantum Dots by Structure-Switching Aptamer for High-Sensitivity Fluorescent Ochratoxin A Sensor. Analytical chemistry 2017, 89 (3), 1704-1709. 12. Zhao, Y.; Yang, Y.; Luo, Y.; Yang, X.; Li, M.; Song, Q., Double Detection of Mycotoxins Based on SERS Labels Embedded Ag@Au Core-Shell Nanoparticles. ACS applied materials & interfaces 2015, 7 (39), 21780-6. 13. Cong, S.; Wang, Z.; Gong, W.; Chen, Z.; Lu, W.; Lombardi, J. R.; Zhao, Z., Electrochromic semiconductors as colorimetric SERS substrates with high reproducibility and renewability. Nature communications 2019, 10 (1), 678. 14. Mao, P.; Liu, C.; Favraud, G.; Chen, Q.; Han, M.; Fratalocchi, A.; Zhang, S., Broadband single molecule SERS detection designed by warped optical spaces. Nature communications 2018, 9 (1), 5428. 15. Zhang, X.; Zhang, X.; Luo, C.; Liu, Z.; Chen, Y.; Dong, S.; Jiang, C.; Yang, S.; Wang, F.; Xiao, X., Volume-Enhanced Raman Scattering Detection of Viruses. Small 2019, 15 (11), e1805516. 16. Tanwar, S.; Haldar, K. K.; Sen, T., DNA Origami Directed Au Nanostar Dimers for Single-Molecule Surface-Enhanced Raman Scattering. Journal of the American Chemical Society 2017, 139 (48), 17639-17648. 17. Whang, K.; Lee, J. H.; Shin, Y.; Lee, W.; Kim, Y. W.; Kim, D.; Lee, L. P.; Kang, T., Plasmonic bacteria on a nanoporous mirror via hydrodynamic trapping for rapid identification of waterborne pathogens. Light, science & applications 2018, 7, 68. 18. Cong, S.; Yuan, Y.; Chen, Z.; Hou, J.; Yang, M.; Su, Y.; Zhang, Y.; Li, L.; Li, Q.; Geng, F.; Zhao, Z., Noble metal-comparable SERS enhancement from semiconducting metal oxides by making oxygen vacancies. Nature communications 2015, 6, 7800. 19. Zhao, Y.; Yang, X.; Li, H.; Luo, Y.; Yu, R.; Zhang, L.; Yang, Y.; Song, Q., Au nanoflower-Ag nanoparticle assembled SERS-active substrates for sensitive MC-LR detection. Chemical communications 2015, 51 (95), 16908-11. 20. Zheng, Z.; Cong, S.; Gong, W.; Xuan, J.; Li, G.; Lu, W.; Geng, F.; Zhao, Z., Semiconductor SERS enhancement enabled by oxygen incorporation. Nature communications 2017, 8 (1), 1993. 21. Dong, J.-C.; Zhang, X.-G.; Briega-Martos, V.; Jin, X.; Yang, J.; Chen, S.; Yang, Z.-L.; Wu, D.-Y.;
Feliu, J. M.; Williams, C. T.;
Tian, Z.-Q.; Li, J.-F., In situ Raman spectroscopic evidence for
oxygen reduction reaction intermediates at platinum single-crystal surfaces. Nature Energy 2018, 4 (1), 60-67. 22. Ma, W.; Sun, M.; Xu, L.; Wang, L.; Kuang, H.; Xu, C., A SERS active gold nanostar dimer for mercury ion detection. Chemical communications 2013, 49 (44), 4989-91. 23. Hu, J.; Wu, T.; Zhang, G.; Liu, S., Efficient synthesis of single gold nanoparticle hybrid amphiphilic triblock copolymers and their controlled self-assembly. Journal of the American Chemical Society 2012, 134 (18), 7624-7. 24. Zhang, W.; Liu, J.; Niu, W.; Yan, H.; Lu, X.; Liu, B., Tip-Selective Growth of Silver on Gold Nanostars for Surface-Enhanced Raman Scattering. ACS applied materials & interfaces 2018, 10 (17), 14850-14856. 25. Su, Y.; Xu, S.; Zhang, J.; Chen, X.; Jiang, L. P.; Zheng, T.; Zhu, J. J., Plasmon Near-Field 14
ACS Paragon Plus Environment
Page 14 of 25
Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
Analytical Chemistry
Coupling of Bimetallic Nanostars and a Hierarchical Bimetallic SERS "Hot Field": Toward Ultrasensitive Simultaneous Detection of Multiple Cardiorenal Syndrome Biomarkers. Analytical chemistry 2019, 91 (1), 864-872. 26. Feng, Y.; Wang, Y.; Song, X.; Xing, S.; Chen, H., Depletion sphere: Explaining the number of Ag islands on Au nanoparticles. Chemical science 2017, 8 (1), 430-436. 27. Feng, Y.; He, J.; Wang, H.; Tay, Y. Y.; Sun, H.; Zhu, L.; Chen, H., An unconventional role of ligand in continuously tuning of metal-metal interfacial strain. Journal of the American Chemical Society 2012, 134 (4), 2004-7. 28. Feng, Y.; Wang, Y.; He, J.; Song, X.; Tay, Y. Y.; Hng, H. H.; Ling, X. Y.; Chen, H., Achieving Site-Specificity in Multistep Colloidal Synthesis. Journal of the American Chemical Society 2015, 137 (24), 7624-7. 29. Zhang, Q.; Li, X.; Ma, Q.; Zhang, Q.; Bai, H.; Yi, W.; Liu, J.; Han, J.; Xi, G., A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy. Nature communications 2017, 8, 14903. 30. Wu, L.; Wang, W.; Zhang, W.; Su, H.; Liu, Q.; Gu, J.; Deng, T.; Zhang, D., Highly sensitive, reproducible and uniform SERS substrates with a high density of three-dimensionally distributed hotspots: gyroid-structured Au periodic metallic materials. NPG Asia Materials 2018, 10 (1), e462. 31. Shi, H.; Chen, N.; Su, Y.; Wang, H.; He, Y., Reusable Silicon-Based Surface-Enhanced Raman Scattering Ratiometric Aptasensor with High Sensitivity, Specificity, and Reproducibility. Analytical chemistry 2017, 89 (19), 10279-10285. 32. Si, Y.; Bai, Y.; Qin, X.; Li, J.; Zhong, W.; Xiao, Z.; Li, J.; Yin, Y., Alkyne-DNA-Functionalized Alloyed Au/Ag Nanospheres for Ratiometric Surface-Enhanced Raman Scattering Imaging Assay of Endonuclease Activity in Live Cells. Analytical chemistry 2018, 90 (6), 3898-3905. 33. Lipatov, A.; Alhabeb, M.; Lukatskaya, M. R.; Boson, A.; Gogotsi, Y.; Sinitskii, A., Effect of Synthesis on Quality, Electronic Properties and Environmental Stability of Individual Monolayer Ti3C2MXene Flakes. Advanced Electronic Materials 2016, 2 (12), 1600255. 34. Zhang, J.; Zhao, Y.; Guo, X.; Chen, C.; Dong, C.-L.; Liu, R.-S.; Han, C.-P.; Li, Y.; Gogotsi, Y.; Wang, G., Single platinum atoms immobilized on an MXene as an efficient catalyst for the hydrogen evolution reaction. Nature Catalysis 2018, 1 (12), 985-992. 35. Hantanasirisakul, K.; Gogotsi, Y., Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides (MXenes). Advanced materials 2018, 30 (52), e1804779. 36. Sarycheva, A.; Makaryan, T.; Maleski, K.; Satheeshkumar, E.; Melikyan, A.; Minassian, H.; Yoshimura, M.; Gogotsi, Y., Two-Dimensional Titanium Carbide (MXene) as Surface-Enhanced Raman Scattering Substrate. The Journal of Physical Chemistry C 2017, 121 (36), 19983-19988. 37. Soundiraraju, B.; George, B. K., Two-Dimensional Titanium Nitride (Ti2N) MXene: Synthesis, Characterization, and Potential Application as Surface-Enhanced Raman Scattering Substrate. ACS nano 2017, 11 (9), 8892-8900. 38. Liu, Y. T.; Zhang, P.; Sun, N.; Anasori, B.; Zhu, Q. Z.; Liu, H.; Gogotsi, Y.; Xu, B., Self-Assembly of Transition Metal Oxide Nanostructures on MXene Nanosheets for Fast and Stable Lithium Storage. Advanced materials 2018, 30 (23), e1707334. 39. Zhang, Q.; Wang, F.; Zhang, H.; Zhang, Y.; Liu, M.; Liu, Y., Universal Ti3C2 MXenes Based Self-Standard Ratiometric Fluorescence Resonance Energy Transfer Platform for Highly Sensitive Detection of Exosomes. Analytical chemistry 2018, 90 (21), 12737-12744. 40. Satheeshkumar, E.; Makaryan, T.; Melikyan, A.; Minassian, H.; Gogotsi, Y.; Yoshimura, M., 15
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3
One-step Solution Processing of Ag, Au and Pd@MXene Hybrids for SERS. Scientific reports 2016, 6, 32049.
4
16
ACS Paragon Plus Environment
Page 16 of 25
Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1 2
Scheme 1. Schematic illustration of the fabrication of Raman IS-aptasensor based on Au-Ag Janus
3
NPs-Mxenes assemblies for the detection of OTA.
4 5
17
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Figure 1. Representative TEM images of Au NPs and Au-Ag Janus NPs (a-h); (a) Au NPs, (b)
3
Au-Ag(1) Janus NPs, (c) Au-Ag(2) Janus NPs, (d) Au-Ag(3) Janus NPs, (e) Au-Ag(4) Janus NPs, (f)
4
Au-Ag(5) Janus NPs, (g) Au-Ag(6) Janus NPs, (h) Au-Ag(7) Janus NPs.
5 6
18
ACS Paragon Plus Environment
Page 18 of 25
Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1 2
Figure 2. (a) The average sizes of Ag islands and the average length to diameter ratio of Au-Ag
3
Janus NPs. (b) UV-vis spectra of prepared Au NPs, Ag NPs and Au-Ag Janus NPs. Insert, from
4
left to right in the photographs are Au NPs, Ag NPs, Au-Ag(1) Janus NPs, Au-Ag(2) Janus NPs,
5
Au-Ag(3) Janus NPs, Au-Ag(4) Janus NPs, Au-Ag(5) Janus NPs, Au-Ag(6) Janus NPs and Au-Ag(7)
6
Janus NPs. (c) SERS spectra of Au@MBIA NPs, Ag@MBIA NPs and Au-Ag Janus NPs. (d) The
7
plot of averaged Raman intensity at 1278 cm-1 of Au@MBIA NPs, Ag@MBIA NPs and Au-Ag
8
Janus NPs.
9 10
19
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Figure 3. (a) TEM images of Mxenes nanosheets. (b) UV-vis spectra of Mxenes nanosheets. (c)
3
Raman spectra of Mxenes nanosheets acquired in twenty continuous measurements at the same
4
spot.
5 6
20
ACS Paragon Plus Environment
Page 20 of 25
Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1 2
Figure 4. Stability of Au-Ag Janus NPs (a) and Mxenes nanosheets (b) under different pH
3
conditions, temperatures and storage times. (c) Corresponding Raman intensities of Au-Ag Janus
4
NPs at 1278 cm-1. (d) Corresponding Raman intensities of Mxenes nanosheets at 730 cm-1.
5
21
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Figure 5. (a) UV-vis spectra of MXenes nanosheets, Au-Ag Janus NPs and Au-Ag Janus
3
NPs-MXenes assemblies. Insert, the photographs of Au-Ag Janus NPs-MXenes assemblies. (b)
4
SERS spectra of MXenes nanosheets, Au-Ag Janus NPs and Au-Ag Janus NPs-MXenes
5
assemblies. (c) SERS spectra of Au-Ag Janus NPs-MXenes assemblies acquired in twenty
6
continuous measurements at the same spot. (d) corresponding RSD results of (c) calculated at
7
I1278/I730.
8 9
22
ACS Paragon Plus Environment
Page 22 of 25
Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1 2
Figure 6. (a) SERS spectra of the SERS aptasensor in detection of OTA with different
3
concentrations. (b) Plots of the SERS intensities of Au-Ag Janus NPs at 1278 cm−1 (red points)
4
and Mxenes nanosheets at 730 cm−1 (blue points) vs logarithmic OTA concentrations and (c)
5
corresponding plot of ratiometric signals (I1278/I730) vs logarithmic OTA concentrations. (d)
6
Ratiometric intensities of I1278/I730 in the presence of 5 nM OTA, BSA, AFB1, FB1 and MC-LR.
7
The blank indicated the solution without any additive.
8 9
23
ACS Paragon Plus Environment
Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Figure 7. (a) The representative SERS spectra of Au-Ag Janus NPs-MXenes assemblies for the
3
detection of OTA samples from Table S2. (b) Statistical analysis of the ratiometric peak intensity
4
under different concentration of OTA.
5 6
24
ACS Paragon Plus Environment
Page 24 of 25
Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
1
For Table of Contents Only
2
25
ACS Paragon Plus Environment