Plasmonic Au-Ag Janus NPs Engineered Ratiometric SERS

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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

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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


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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



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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

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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


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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



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


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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



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


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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



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


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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



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


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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

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One-step Solution Processing of Ag, Au and Pd@MXene Hybrids for SERS. Scientific reports 2016, 6, 32049.

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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



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

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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



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.

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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



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

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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



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

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For Table of Contents Only

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Plasmonic Au-Ag Janus NPs Engineered Ratiometric SERS

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