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Few-layer Ti3C2Tx MXene: A promising surface plasmon resonance biosensing material to enhance the sensitivity
Accepted Manuscript Title: Few-layer Ti3 C2 Tx MXene: a promising surface plasmon resonance biosensing material to enhance the sensitivity Authors: Leiming Wu, Qi You, Youxian Shan, Shuaiwen Gan, Yuting Zhao, Xiaoyu Dai, Yuanjiang Xiang PII: DOI: Reference:
S0925-4005(18)31592-2 https://doi.org/10.1016/j.snb.2018.08.154 SNB 25302
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4-6-2018 24-8-2018 29-8-2018
Please cite this article as: Wu L, You Q, Shan Y, Gan S, Zhao Y, Dai X, Xiang Y, Few-layer Ti3 C2 Tx MXene: a promising surface plasmon resonance biosensing material to enhance the sensitivity, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.08.154 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Few-layer Ti3C2Tx MXene: a promising surface plasmon
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resonance biosensing material to enhance the sensitivity
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Leiming Wu1, 2, 3, Qi You1, 2, Youxian Shan1, 2, Shuaiwen Gan1, 2, Yuting Zhao1, 2, Xiaoyu Dai1, 2, *, Yuanjiang Xiang1, 2
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of Information Technology, Macau University of Science and Technology, Macao 519020, China.
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3Faculty
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International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technoloy of Ministry of Education , College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. 2Laboratory of advanced material photonics (LAMPs), Shenzhen University, Shenzhen 518060, China.
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*Corresponding Author: [email protected]
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Graphical Abstract
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Sensitivity enhanced by Ti3C2Tx MXene in SPR biosensors. High sensitivity (224.5 °/RIU) is obtained. Highest sensitivity enhancement ~ 46.3% for Al- Ti3C2Tx MXene-based SPR biosensor. Be able to detect the analyte with different refractive index.
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Highlights
Abstract:
In this contribution, a novel surface plasmon resonance (SPR) biosensor with few-layer Ti3C2Tx MXene to enhance the sensitivity is proposed. Few-layer Ti3C2Tx MXene is coated on the surface of metals thin film to
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act as a protective layer and further to improve the sensitivity. The results show that the sensitivity enhancements at λ = 633 nm are 16.8%, 28.4%, 46.3% and 33.6% for the proposed SPR biosensors based on Au with 4 layers Ti3C2Tx, Ag with 7 layers Ti3C2Tx, Al with 12 layers Ti3C2Tx and Cu with 9 layers Ti3C2Tx, respectively. Moreover, we have discussed the sensitivity of the proposed Au-based SPR biosensor with monolayer Ti3C2Tx MXene works at λ=532 nm, the result shows that the sensitivity can reach 224.5 °/RIU.
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Our contributions reveal potential applications of few-layer Ti3C2Tx MXene as a new type of biosensing material, and it is therefore anticipated that Ti3C2Tx MXene and other 2D MXene nanomaterials could find promising applications in SPR biosensors.
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Keywords: few-layer Ti3C2Tx MXene; surface plasmon resonance; biosensors; sensitivity
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1. Introduction
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Few-layer Ti3C2Tx MXene [1-3], a new biosensing material [4, 5], has attracted widespread attention in
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recent years. The abundance of functional groups on the surface of Ti3C2Tx MXene gives it the band gapadjustable performance. MXene is an environmentally benign two-dimensional (2D) material with strong
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hydrophilic due to its unique layered structure and chemical stability. Moreover, MXene shows excellent adsorption properties with a large specific surface area [6, 7], which makes it has a good promising
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application in biosensing technology [8, 9]. Yu et al. [10] demonstrated that monolayer MXene can selectively adsorb NH3 by using the first-principle principle, so that a single layer of MXene can be used for NH3 sensors. Chen et al. [11] confirmed that V2C MXene V-grafted polyethyl methacrylate (PDMAEMA) is
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sensitive to CO2 and temperature, so that it can be applied in biosensors. In addition, Liu [12] et al. proposed a novel nitrite biosensor based on the direct electrochemistry of hemoglobin immobilized on Ti3C2Tx MXene, and it displayed the good performance for the detection of nitrite with a wide linear range of 0.5-11800 μM, as well as an extremely low detection limit of 0.12 μM. These reports show that MXene can not only be used for chemical sensation, but also be used in biological sensation.
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SPR signal is extremely sensitive to the change of surrounding environment, and it has been widely used in sensing technology [13-15]. The phenomena of SPR based on Kretschmann configuration occurs [16] when the electromagnetic waves coupled to collective electron oscillations, and then the surface plasmon wave is propagating and exponentially decaying along the interface of metal-dielectric. In biosensors, SPR signal is used to detect the occurrence of a biological action or chemical reaction. With the development of
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materials science, many new 2D materials have been discovered and widely used in biosensing technology in recent years, such as graphene [17, 18], MoS2 [19, 20], and black phosphorous (BP) [21, 22], etc. Wu et al. [23] have reported that the sensitivity of a SPR biosensor can be nearly 25％ enhanced with ten layers of graphene. Gupta et al. [24] have presented a SPR biosensor with graphene and silicon to enhance the sensitivity and a maximum sensitivity ~ 134.6 º/RIU is achieved. Ouyang et al. [25] demonstrated a SPR
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biosensor by using MoS2 and silicon to enhance the sensitivity. They obtained the highest sensitivity ~
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125.44º/RIU.
In this article, we propose a SPR biosensor by using few-layer Ti3C2Tx MXene to enhance the sensitivity.
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Few-layer Ti3C2Tx MXene is coated on the surface of metals thin film to act as a protective layer and further
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to improve the sensitivity. Our contributions reveal potential applications of multilayer Ti3C2Tx MXene as a new type of biosensing material, and it is therefore anticipated that Ti3C2Tx MXene and other 2D MXene
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nanomaterials could find promising applications in SPR biosensors.
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2. Design consideration and theoretical model The proposed SPR biosensor by using few-layer Ti3C2Tx MXene to enhance the sensitivity is shown in Figure
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1. To obtain a higher sensitivity, BK7 is elected as the coupling prism to be used in the proposed MXene-
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based SPR biosensor and its refractive index (RI) is calculated from the following relation [26]: 1/2
,
(1)
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1.03961212 2 0.231792344 2 1.03961212 2 nBk 7 = 2 2 2 1 0.00600069867 0.0200179144 103.560653
where is the wavelength of incident light. Next, according to the Drude model, the dielectric constant of metal layer is given as [16]:
m 1
2c , p2 c i
(2)
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where p and c represent the plasma and collision wavelength. The values of p for silver (Ag), gold (Au), aluminum (Al) and copper (Cu) are 1.4541107 m , 1.6826 107 m , 1.0657 107 m and 1.3617 107 m , respectively; and the values of c for Ag, Au, Al and Cu are 1.7614 105 m , 8.9342 106 m , 2.4511105 m and 4.0852 105 m , respectively. The thickness of the metal layer is 50 nm. Furthermore, the RI of few-
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layer Ti3C2Tx MXene in visible range is given in table 1 [27], and the thickness of monolayer Ti3C2Tx MXene is 0.993 nm [28]. Finally, the last layer is sensing medium, and the RI is given as ns=1.33+Δn, where Δn is the change of RI in sensing medium due to the occurrence of a biological action or chemical reaction.
In the proposed structure, all layers stack along in the z-direction and each layer is defined by their thickness (dk), refractive index (nk) or dielectric constant ( k ). The tangential fields at the first and the final
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boundary are set as Z Z1 0 and Z Z N 1 , respectively, and their relation is given as [29]:
(3)
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U N 1 U1 , M V1 VN 1
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where U1 and V1 are respectively the electric and magnetic fields at the boundary of first layer, U N 1 and VN 1 are respectively the electric and magnetic at the boundary of Nth layer. M is the characteristic transfer
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matrix of the combined N-layer structure, and M is given as:
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M11M12 N 1 M k 2 M k , M 21M 22
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with
cos k i sin k qk M k , i q sin cos k k k
where θ1 is the incident angle, qk k n12 sin 2 1
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(4)
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k for TM-polarized mode. Appling the transfer matrix
method for N-layer model, we can obtain four elements M11, M12, M21, and M22 of M. Using these elements to calculate the total reflection coefficient rp, we can get the relation as: rp
M11 M12 qN q1 M 21 M 22 qN . M11 M12 qN q1 M 21 M 22 qN
(6)
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Finally, the reflectance (Rp) of N-layer model is given by 2
R rp .
(7)
The change in the RI of the sensing medium (Δns) can lead to a change in resonance angle (Δθ). Hence the
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sensitivity is given as [21, 30]: , ns
3. Results and discussions
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In the biosensors, SPR is usually used as the detection signal to detect the occurrence of a biological action
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or chemical reaction. In the conventional SPR sensor, Au film is applied as the SPPs material to generate SPR. However, the conventional Au-based SPR sensor still has a deficiency that its sensitivity not high
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enough to detect more slight changes in the surrounding environment. The variation of reflectance with
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respect to the incident angle for the conventional SPR sensor with a 50 nm thick Au has been shown in
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Figure 2(a). When the RI of sensing medium change from 1.330 to 1.335 (Δns=0.005), the change in resonance angle is 0.685° and its sensitivity is 137 °/RIU. In order to improve the sensitivity for the SPR
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biosensors, few-layer Ti3C2Tx MXene (L=4) is coated on the surface of Au film, as the structure shows inside
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Figure 2(b). By calculation, we can find that the shift of the resonance angle is 0.800° when the change in RI of sensing medium is fixed as Δns=0.005 (Figure 2(b)), this means that its sensitivity has increased to
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160 °/RIU.
The performance of the proposed SPR biosensor is greatly affected by the number of Ti3C2Tx MXene
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layers. Figure 3 shows the variation of reflectance, change in resonance angle (Δθ), sensitivity and sensitivity enhancement ((SL-S0)/S0) with respect to the different number of Ti3C2Tx MXene layers at λ=633 nm. Fig. 3(a) illustrates that with the number of Ti3C2Tx layers increasing, the SPR curves become broader because
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of the increased electron energy loss of the increased number of Ti3C2Tx layers, as reported for graphene [31]. Moreover, the resonance angle has an obvious movement with the increase of the Ti3C2Tx layers due to the increase of absorption. It is known that Δθ is strongly dependent on the number of Ti3C2Tx layers when Δns=0.005, as shown in Figure 3(b). Figure 3(c) shows that there is a dramatic increase in the sensitivity with the number of Ti3C2Tx layers varying from 0 to 4, and the largest sensitivity (160 °/RIU) can be obtained for L=4. After that, sensitivity begins to decrease for N>4. Figure 3(d) shows the sensitivity
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enhancement for the proposed Au-based SPR sensor with different number of Ti3C2Tx layers, when L=4, the highest sensitivity enhancement is 16.8%. It must be noted that the range of incident angle is 0o~90o. With the increase of Ti3C2Tx layer number, the reflectance curve will shift to a larger angle and its limit is 90 o. When the number of Ti3C2Tx layer is L=4, we can obtain the largest change in resonance angle to receive the highest sensitivity. However, when the number of Ti3C2Tx layer is large than four layers, the sensitivity
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begins to decrease due to the limitation of the angle range.
Metals, other than Au, such as Ag, Al, and Cu can also be used for SPR sensing. However, these metals are easily oxidized to weaken the sensitivity. In order to prevent these metals from being oxidized, fewlayer Ti3C2Tx Mxene is coated on the surface of metal thin films to act as the protective layer and further improve the sensitivity. Figure 4 shows the sensitivity and sensitivity enhancement of the proposed SPR
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biosensor based on Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx, respectively. The figure clearly demonstrates that
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coating suitable Ti3C2Tx Mxene layer can effectively improve the sensitivity of the proposed SPR biosensor. The sensitivity enhancements are 28.4%, 46.3% and 33.6% for the proposed SPR biosensors based on Ag
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with 7L Ti3C2Tx, Al with 12L Ti3C2Tx and Cu with 9L Ti3C2Tx, respectively. Table 2 shows the optimized layers,
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highest sensitivity and the sensitivity enhancement for the proposed SPR biosensor based on different metals and few-layer Ti3C2Tx hybrid nanostructures at λ=633 nm. Similar to the Au- Ti3C2Tx based SPR sensor,
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the sensitivity of Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx based SPR sensors will firstly increase to the largest
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one at the optimal number of Ti3C2Tx layers. However, when the number of Ti3C2Tx layer is large than the optimal number, the sensitivity begins to decrease due to the limitation of the angle range.
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Figure 5 has discussed the sensitivity of the proposed Au-based SPR biosensor with monolayer Ti3C2Tx MXene. Figure 5(a) shows the shift of SPR signal when the RI of sensing medium has a slight change
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(Δns=0.005), and Figure 5(b) illustrates the movement of the corresponding resonance angle (θr) with respect to the RI of sensing medium change from 1.32 to 1.36. We have a comparison of the proposed Au-
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Ti3C2Tx MXene-based SPR biosensor to the conventional Au-based biosensor (Figure 5(a)). The result shows that the sensitivity of the proposed Au-Ti3C2Tx MXene-based SPR biosensor can obtain a larger sensitivity than the conventional Au-based biosensor when the RI of sensing medium change from 1.33 to 1.36. Moreover, we have discussed the change of electric field for the proposed SPR biosensor with monolayer Ti3C2Tx MXene when the RI of sensing medium has a slight change (Figure 5(d)). It illustrates that the electric field at the interface of Ti3C2Tx MXene and sensing medium can have a dramatic change with a slight change in RI of sensing medium. Here we only give an example to discuss the high sensitivity of the Au-Ti3C2Tx
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MXene-based SPR biosensor, but other structures, such as Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx-based biosensors, can also have a high sensitivity when the RI of sensing medium changes. The above discussions for the proposed SPR biosensors are working at λ=633 nm. However, the proposed SPR biosensor can also work in other wavelength. Figure 6 we have shown the proposed Au-based SPR
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biosensor with monolayer Ti3C2Tx MXene that works at λ=532 nm. Figure 6(a) shows the variation of the reflectance with respect to the incident angle when the RI of sensing medium change from 1.325 to 1.335 (Δns=0.002) at λ=532 nm. Although the change in RI of sensing medium is very slight, our proposed SPR sensor can also effectively to have the detection. Figure 6(b) shows the movement of θr for the proposed Au-Ti3C2Tx MXene-based SPR biosensor when the RI of sensing medium has an increase and the θs is the resonance deviation angle. Here, we can write the sensitivity as: S=tanθs (Figure 6(c)). Figure 6(c) shows the
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variation of Δθ with respect to the RI of sensing medium. After the calculation, we can obtain a high
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sensitivity (225.4 °/RIU) of the proposed Au-Ti3C2Tx MXene-based SPR biosensor at λ=532 nm. Moreover, the electric field distributions for proposed Au-based biosensor with monolayer Ti3C2Tx MXene at λ=532 nm
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have also been discussed (Figure 6(d)). When the RI of sensing medium has a slight change from 1.325 to
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1.335, the electric field at the interface of Ti3C2Tx MXene and sensing medium can has an obvious change,
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which demonstrates the high sensitivity for the proposed SPR biosensor.
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4. Conclusions
In this work, a novel surface plasmon resonance (SPR) biosensor is proposed by using few-layer Ti3C2Tx
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MXene to enhance the sensitivity. It is shown that coating few-layer Ti3C2Tx MXene on the surface of SPR biosensors can improve the sensitivity due to its absorption. The most prominent advantage of the proposed structure is its high sensitivity. The sensitivity enhancements are 16.8%, 28.4%, 46.3% and 33.6%
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for the proposed SPR biosensors based on Au with 4 layers Ti3C2Tx, Ag with 7 layers Ti3C2Tx, Al with 12 layers Ti3C2Tx and Cu with 9 layers Ti3C2Tx, respectively. With such outstanding performance, we believe that the
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proposed metals- Ti3C2Tx MXene-based SPR biosensors can play an important role in sensing technology.
ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant Nos. 61875133, 11874269, 61505111 and 11604216), the China Postdoctoral Science Foundation (Grant Nos. 2017M622746 and 2018M633129), and the Guangdong Natural Science Foundation (Grant No. 2018A030313198).
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Biographies
Leiming Wu is currently a postgraduate at college of Faculty of Information Technology, Macau
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University of Science and Technology, Macao, China. His main research interests focus on enhancing the sensitivity of surface plasmon resonance sensor.
Qi You is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon resonance sensor.
Youxian Shan is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
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Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon
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resonance sensor.
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Shuaiwen Gan is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
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Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon resonance sensor.
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Yuting Zhao is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
resonance sensor.
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Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon
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Xiaoyu Dai received her Ph.D. degree in technology of computer application from Hunan University, Changsha, China, in 2009. She is currently an Associate Professor with Shenzhen University, Shenzhen,
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China. Her main research interests include theory of nonlinear metamaterials and optical solitons and interaction of electromagnetic waves with matter.
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Yuanjiang Xiang received his Ph.D. degree in electronic circuit and system from Hunan University, Changsha, China, in 2011. He is currently an Associate Professor with Shenzhen University, Shenzhen, China. His main research interests include theory and simulation of nonlinear metamaterials and photonic crystals, theory of optical solitons, and characterization of high performance optical switcher and optical filter for optical communications.
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Figure Captions
Figure 1 Schematic diagram of the proposed SPR biosensor by using few-layer Ti3C2Tx MXene to enhance the sensitivity.
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Figure 2 Variation of the reflectance with respect to the incident angle for (a) the conventional biosensor based on single Au film and (b) the proposed biosensor with few-layer Ti3C2Tx MXene.
Figure 3 Variation of (a) reflectance, (b) change in resonance angle (Δθ), (c) sensitivity and (d) sensitivity enhancement ((SL-S0)/S0) with respect to the different number of Ti3C2Tx MXene layers at λ=633 nm. Figure 4 (a-c) shows the change of sensitivity with the increase of Ti3C2Tx MXene layers at λ=633 nm for the
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proposed biosensors based on Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx, respectively; (d-f) shows the variation
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of the sensitivity enhancement with respect to the layers of Ti3C2Tx MXene at λ=633 nm for the proposed
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biosensors based on Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx, respectively.
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Figure 5 (a) Variation of the reflectance with respect to the incident angle when the RI of sensing medium change from 1.33 to 1.36 (Δns=0.005) for the proposed Au-based biosensor with monolayer Ti3C2Tx MXene
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at λ=633 nm; (b) Movement of resonance angle (θr) for the proposed Au-Ti3C2Tx MXene-based SPR biosensor when the RI of sensing medium has an increase; (c) Variation of sensitivity with respect to the RI
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of sensing medium for the proposed Au-based biosensor with different number of Ti3C2Tx MXene layers at λ=633 nm; (d) The electric field distributions for the proposed SPR biosensor with monolayer Ti3C2Tx MXene
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when the RI of sensing medium has a change.
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Figure 6 (a) Variation of the reflectance with respect to the incident angle when the RI of sensing medium change from 1.325 to 1.335 (Δns=0.002) for the proposed Au-based biosensor with monolayer Ti3C2Tx MXene at λ=532 nm; (b) Movement of θr for the proposed Au-Ti3C2Tx MXene-based SPR biosensor when
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the RI of sensing medium has an increase and the θs is the resonance deviation angle; (c) Variation of Δθ with respect to the RI of sensing medium; (d) The electric field distributions for proposed Au-based biosensor with monolayer Ti3C2Tx MXene at λ=532 nm when the RI of sensing medium has a slight change.
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Table 1 RI of monolayer Ti3C2Tx MXene at visible range (nMXene=n+ik) [27].
633 nm
2.38
532 nm
2.64
k
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n
1.33 1.00
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Wavelength
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Table 2 The optimized layers, highest sensitivity and the sensitivity enhancement for the proposed SPR biosensor based on different metals and few-layer Ti3C2Tx hybrid nanostructures at λ=633 nm. Type of the
Sensitivity enhancement Highest sensitivity (°/RIU)
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Optimized Layers (L) biosensors
((SL-S0)/S0)
160
Ag+Ti3C2Tx
7
149
Al+Ti3C2Tx
12
139
Cu+Ti3C2Tx
9
147
16.8%
28.4% 46.3%
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4
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Au+Ti3C2Tx
33.6%
S0925-4005(18)31592-2 https://doi.org/10.1016/j.snb.2018.08.154 SNB 25302
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4-6-2018 24-8-2018 29-8-2018
Please cite this article as: Wu L, You Q, Shan Y, Gan S, Zhao Y, Dai X, Xiang Y, Few-layer Ti3 C2 Tx MXene: a promising surface plasmon resonance biosensing material to enhance the sensitivity, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.08.154 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sensors and Actuators B: Chemical
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Few-layer Ti3C2Tx MXene: a promising surface plasmon
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resonance biosensing material to enhance the sensitivity
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Leiming Wu1, 2, 3, Qi You1, 2, Youxian Shan1, 2, Shuaiwen Gan1, 2, Yuting Zhao1, 2, Xiaoyu Dai1, 2, *, Yuanjiang Xiang1, 2
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of Information Technology, Macau University of Science and Technology, Macao 519020, China.
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3Faculty
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International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technoloy of Ministry of Education , College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. 2Laboratory of advanced material photonics (LAMPs), Shenzhen University, Shenzhen 518060, China.
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*Corresponding Author: [email protected]
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Graphical Abstract
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Sensitivity enhanced by Ti3C2Tx MXene in SPR biosensors. High sensitivity (224.5 °/RIU) is obtained. Highest sensitivity enhancement ~ 46.3% for Al- Ti3C2Tx MXene-based SPR biosensor. Be able to detect the analyte with different refractive index.
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Highlights
Abstract:
In this contribution, a novel surface plasmon resonance (SPR) biosensor with few-layer Ti3C2Tx MXene to enhance the sensitivity is proposed. Few-layer Ti3C2Tx MXene is coated on the surface of metals thin film to
Sensors and Actuators B: Chemical
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act as a protective layer and further to improve the sensitivity. The results show that the sensitivity enhancements at λ = 633 nm are 16.8%, 28.4%, 46.3% and 33.6% for the proposed SPR biosensors based on Au with 4 layers Ti3C2Tx, Ag with 7 layers Ti3C2Tx, Al with 12 layers Ti3C2Tx and Cu with 9 layers Ti3C2Tx, respectively. Moreover, we have discussed the sensitivity of the proposed Au-based SPR biosensor with monolayer Ti3C2Tx MXene works at λ=532 nm, the result shows that the sensitivity can reach 224.5 °/RIU.
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Our contributions reveal potential applications of few-layer Ti3C2Tx MXene as a new type of biosensing material, and it is therefore anticipated that Ti3C2Tx MXene and other 2D MXene nanomaterials could find promising applications in SPR biosensors.
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Keywords: few-layer Ti3C2Tx MXene; surface plasmon resonance; biosensors; sensitivity
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1. Introduction
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Few-layer Ti3C2Tx MXene [1-3], a new biosensing material [4, 5], has attracted widespread attention in
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recent years. The abundance of functional groups on the surface of Ti3C2Tx MXene gives it the band gapadjustable performance. MXene is an environmentally benign two-dimensional (2D) material with strong
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hydrophilic due to its unique layered structure and chemical stability. Moreover, MXene shows excellent adsorption properties with a large specific surface area [6, 7], which makes it has a good promising
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application in biosensing technology [8, 9]. Yu et al. [10] demonstrated that monolayer MXene can selectively adsorb NH3 by using the first-principle principle, so that a single layer of MXene can be used for NH3 sensors. Chen et al. [11] confirmed that V2C MXene V-grafted polyethyl methacrylate (PDMAEMA) is
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sensitive to CO2 and temperature, so that it can be applied in biosensors. In addition, Liu [12] et al. proposed a novel nitrite biosensor based on the direct electrochemistry of hemoglobin immobilized on Ti3C2Tx MXene, and it displayed the good performance for the detection of nitrite with a wide linear range of 0.5-11800 μM, as well as an extremely low detection limit of 0.12 μM. These reports show that MXene can not only be used for chemical sensation, but also be used in biological sensation.
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SPR signal is extremely sensitive to the change of surrounding environment, and it has been widely used in sensing technology [13-15]. The phenomena of SPR based on Kretschmann configuration occurs [16] when the electromagnetic waves coupled to collective electron oscillations, and then the surface plasmon wave is propagating and exponentially decaying along the interface of metal-dielectric. In biosensors, SPR signal is used to detect the occurrence of a biological action or chemical reaction. With the development of
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materials science, many new 2D materials have been discovered and widely used in biosensing technology in recent years, such as graphene [17, 18], MoS2 [19, 20], and black phosphorous (BP) [21, 22], etc. Wu et al. [23] have reported that the sensitivity of a SPR biosensor can be nearly 25％ enhanced with ten layers of graphene. Gupta et al. [24] have presented a SPR biosensor with graphene and silicon to enhance the sensitivity and a maximum sensitivity ~ 134.6 º/RIU is achieved. Ouyang et al. [25] demonstrated a SPR
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biosensor by using MoS2 and silicon to enhance the sensitivity. They obtained the highest sensitivity ~
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125.44º/RIU.
In this article, we propose a SPR biosensor by using few-layer Ti3C2Tx MXene to enhance the sensitivity.
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Few-layer Ti3C2Tx MXene is coated on the surface of metals thin film to act as a protective layer and further
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to improve the sensitivity. Our contributions reveal potential applications of multilayer Ti3C2Tx MXene as a new type of biosensing material, and it is therefore anticipated that Ti3C2Tx MXene and other 2D MXene
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nanomaterials could find promising applications in SPR biosensors.
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2. Design consideration and theoretical model The proposed SPR biosensor by using few-layer Ti3C2Tx MXene to enhance the sensitivity is shown in Figure
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1. To obtain a higher sensitivity, BK7 is elected as the coupling prism to be used in the proposed MXene-
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based SPR biosensor and its refractive index (RI) is calculated from the following relation [26]: 1/2
,
(1)
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1.03961212 2 0.231792344 2 1.03961212 2 nBk 7 = 2 2 2 1 0.00600069867 0.0200179144 103.560653
where is the wavelength of incident light. Next, according to the Drude model, the dielectric constant of metal layer is given as [16]:
m 1
2c , p2 c i
(2)
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where p and c represent the plasma and collision wavelength. The values of p for silver (Ag), gold (Au), aluminum (Al) and copper (Cu) are 1.4541107 m , 1.6826 107 m , 1.0657 107 m and 1.3617 107 m , respectively; and the values of c for Ag, Au, Al and Cu are 1.7614 105 m , 8.9342 106 m , 2.4511105 m and 4.0852 105 m , respectively. The thickness of the metal layer is 50 nm. Furthermore, the RI of few-
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layer Ti3C2Tx MXene in visible range is given in table 1 [27], and the thickness of monolayer Ti3C2Tx MXene is 0.993 nm [28]. Finally, the last layer is sensing medium, and the RI is given as ns=1.33+Δn, where Δn is the change of RI in sensing medium due to the occurrence of a biological action or chemical reaction.
In the proposed structure, all layers stack along in the z-direction and each layer is defined by their thickness (dk), refractive index (nk) or dielectric constant ( k ). The tangential fields at the first and the final
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boundary are set as Z Z1 0 and Z Z N 1 , respectively, and their relation is given as [29]:
(3)
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U N 1 U1 , M V1 VN 1
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where U1 and V1 are respectively the electric and magnetic fields at the boundary of first layer, U N 1 and VN 1 are respectively the electric and magnetic at the boundary of Nth layer. M is the characteristic transfer
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matrix of the combined N-layer structure, and M is given as:
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M11M12 N 1 M k 2 M k , M 21M 22
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with
cos k i sin k qk M k , i q sin cos k k k
where θ1 is the incident angle, qk k n12 sin 2 1
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12
(4)
(5)
k for TM-polarized mode. Appling the transfer matrix
method for N-layer model, we can obtain four elements M11, M12, M21, and M22 of M. Using these elements to calculate the total reflection coefficient rp, we can get the relation as: rp
M11 M12 qN q1 M 21 M 22 qN . M11 M12 qN q1 M 21 M 22 qN
(6)
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Finally, the reflectance (Rp) of N-layer model is given by 2
R rp .
(7)
The change in the RI of the sensing medium (Δns) can lead to a change in resonance angle (Δθ). Hence the
S
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sensitivity is given as [21, 30]: , ns
3. Results and discussions
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In the biosensors, SPR is usually used as the detection signal to detect the occurrence of a biological action
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or chemical reaction. In the conventional SPR sensor, Au film is applied as the SPPs material to generate SPR. However, the conventional Au-based SPR sensor still has a deficiency that its sensitivity not high
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enough to detect more slight changes in the surrounding environment. The variation of reflectance with
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respect to the incident angle for the conventional SPR sensor with a 50 nm thick Au has been shown in
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Figure 2(a). When the RI of sensing medium change from 1.330 to 1.335 (Δns=0.005), the change in resonance angle is 0.685° and its sensitivity is 137 °/RIU. In order to improve the sensitivity for the SPR
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biosensors, few-layer Ti3C2Tx MXene (L=4) is coated on the surface of Au film, as the structure shows inside
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Figure 2(b). By calculation, we can find that the shift of the resonance angle is 0.800° when the change in RI of sensing medium is fixed as Δns=0.005 (Figure 2(b)), this means that its sensitivity has increased to
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160 °/RIU.
The performance of the proposed SPR biosensor is greatly affected by the number of Ti3C2Tx MXene
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layers. Figure 3 shows the variation of reflectance, change in resonance angle (Δθ), sensitivity and sensitivity enhancement ((SL-S0)/S0) with respect to the different number of Ti3C2Tx MXene layers at λ=633 nm. Fig. 3(a) illustrates that with the number of Ti3C2Tx layers increasing, the SPR curves become broader because
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of the increased electron energy loss of the increased number of Ti3C2Tx layers, as reported for graphene [31]. Moreover, the resonance angle has an obvious movement with the increase of the Ti3C2Tx layers due to the increase of absorption. It is known that Δθ is strongly dependent on the number of Ti3C2Tx layers when Δns=0.005, as shown in Figure 3(b). Figure 3(c) shows that there is a dramatic increase in the sensitivity with the number of Ti3C2Tx layers varying from 0 to 4, and the largest sensitivity (160 °/RIU) can be obtained for L=4. After that, sensitivity begins to decrease for N>4. Figure 3(d) shows the sensitivity
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enhancement for the proposed Au-based SPR sensor with different number of Ti3C2Tx layers, when L=4, the highest sensitivity enhancement is 16.8%. It must be noted that the range of incident angle is 0o~90o. With the increase of Ti3C2Tx layer number, the reflectance curve will shift to a larger angle and its limit is 90 o. When the number of Ti3C2Tx layer is L=4, we can obtain the largest change in resonance angle to receive the highest sensitivity. However, when the number of Ti3C2Tx layer is large than four layers, the sensitivity
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begins to decrease due to the limitation of the angle range.
Metals, other than Au, such as Ag, Al, and Cu can also be used for SPR sensing. However, these metals are easily oxidized to weaken the sensitivity. In order to prevent these metals from being oxidized, fewlayer Ti3C2Tx Mxene is coated on the surface of metal thin films to act as the protective layer and further improve the sensitivity. Figure 4 shows the sensitivity and sensitivity enhancement of the proposed SPR
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biosensor based on Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx, respectively. The figure clearly demonstrates that
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coating suitable Ti3C2Tx Mxene layer can effectively improve the sensitivity of the proposed SPR biosensor. The sensitivity enhancements are 28.4%, 46.3% and 33.6% for the proposed SPR biosensors based on Ag
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with 7L Ti3C2Tx, Al with 12L Ti3C2Tx and Cu with 9L Ti3C2Tx, respectively. Table 2 shows the optimized layers,
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highest sensitivity and the sensitivity enhancement for the proposed SPR biosensor based on different metals and few-layer Ti3C2Tx hybrid nanostructures at λ=633 nm. Similar to the Au- Ti3C2Tx based SPR sensor,
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the sensitivity of Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx based SPR sensors will firstly increase to the largest
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one at the optimal number of Ti3C2Tx layers. However, when the number of Ti3C2Tx layer is large than the optimal number, the sensitivity begins to decrease due to the limitation of the angle range.
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Figure 5 has discussed the sensitivity of the proposed Au-based SPR biosensor with monolayer Ti3C2Tx MXene. Figure 5(a) shows the shift of SPR signal when the RI of sensing medium has a slight change
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(Δns=0.005), and Figure 5(b) illustrates the movement of the corresponding resonance angle (θr) with respect to the RI of sensing medium change from 1.32 to 1.36. We have a comparison of the proposed Au-
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Ti3C2Tx MXene-based SPR biosensor to the conventional Au-based biosensor (Figure 5(a)). The result shows that the sensitivity of the proposed Au-Ti3C2Tx MXene-based SPR biosensor can obtain a larger sensitivity than the conventional Au-based biosensor when the RI of sensing medium change from 1.33 to 1.36. Moreover, we have discussed the change of electric field for the proposed SPR biosensor with monolayer Ti3C2Tx MXene when the RI of sensing medium has a slight change (Figure 5(d)). It illustrates that the electric field at the interface of Ti3C2Tx MXene and sensing medium can have a dramatic change with a slight change in RI of sensing medium. Here we only give an example to discuss the high sensitivity of the Au-Ti3C2Tx
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MXene-based SPR biosensor, but other structures, such as Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx-based biosensors, can also have a high sensitivity when the RI of sensing medium changes. The above discussions for the proposed SPR biosensors are working at λ=633 nm. However, the proposed SPR biosensor can also work in other wavelength. Figure 6 we have shown the proposed Au-based SPR
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biosensor with monolayer Ti3C2Tx MXene that works at λ=532 nm. Figure 6(a) shows the variation of the reflectance with respect to the incident angle when the RI of sensing medium change from 1.325 to 1.335 (Δns=0.002) at λ=532 nm. Although the change in RI of sensing medium is very slight, our proposed SPR sensor can also effectively to have the detection. Figure 6(b) shows the movement of θr for the proposed Au-Ti3C2Tx MXene-based SPR biosensor when the RI of sensing medium has an increase and the θs is the resonance deviation angle. Here, we can write the sensitivity as: S=tanθs (Figure 6(c)). Figure 6(c) shows the
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variation of Δθ with respect to the RI of sensing medium. After the calculation, we can obtain a high
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sensitivity (225.4 °/RIU) of the proposed Au-Ti3C2Tx MXene-based SPR biosensor at λ=532 nm. Moreover, the electric field distributions for proposed Au-based biosensor with monolayer Ti3C2Tx MXene at λ=532 nm
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have also been discussed (Figure 6(d)). When the RI of sensing medium has a slight change from 1.325 to
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1.335, the electric field at the interface of Ti3C2Tx MXene and sensing medium can has an obvious change,
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which demonstrates the high sensitivity for the proposed SPR biosensor.
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4. Conclusions
In this work, a novel surface plasmon resonance (SPR) biosensor is proposed by using few-layer Ti3C2Tx
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MXene to enhance the sensitivity. It is shown that coating few-layer Ti3C2Tx MXene on the surface of SPR biosensors can improve the sensitivity due to its absorption. The most prominent advantage of the proposed structure is its high sensitivity. The sensitivity enhancements are 16.8%, 28.4%, 46.3% and 33.6%
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for the proposed SPR biosensors based on Au with 4 layers Ti3C2Tx, Ag with 7 layers Ti3C2Tx, Al with 12 layers Ti3C2Tx and Cu with 9 layers Ti3C2Tx, respectively. With such outstanding performance, we believe that the
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proposed metals- Ti3C2Tx MXene-based SPR biosensors can play an important role in sensing technology.
ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant Nos. 61875133, 11874269, 61505111 and 11604216), the China Postdoctoral Science Foundation (Grant Nos. 2017M622746 and 2018M633129), and the Guangdong Natural Science Foundation (Grant No. 2018A030313198).
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based surface plasmon resonance biosensor for near infrared, Sens. Actuators B 190(1) (2014) 494-501. S. H. Choi, Y. L. Kim, K. M. Byun, Graphene-on-silver substrates for sensitive surface plasmon
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[31]
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resonance imaging biosensors, Opt. Express 19(12) (2011) 458-66.
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Biographies
Leiming Wu is currently a postgraduate at college of Faculty of Information Technology, Macau
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University of Science and Technology, Macao, China. His main research interests focus on enhancing the sensitivity of surface plasmon resonance sensor.
Qi You is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon resonance sensor.
Youxian Shan is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
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Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon
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resonance sensor.
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Shuaiwen Gan is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
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Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon resonance sensor.
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Yuting Zhao is currently a postgraduate at college of optoelectronic engineering, Shenzhen University,
resonance sensor.
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Shenzhen, China. His main research interests focus on enhancing the sensitivity of surface plasmon
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Xiaoyu Dai received her Ph.D. degree in technology of computer application from Hunan University, Changsha, China, in 2009. She is currently an Associate Professor with Shenzhen University, Shenzhen,
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China. Her main research interests include theory of nonlinear metamaterials and optical solitons and interaction of electromagnetic waves with matter.
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Yuanjiang Xiang received his Ph.D. degree in electronic circuit and system from Hunan University, Changsha, China, in 2011. He is currently an Associate Professor with Shenzhen University, Shenzhen, China. His main research interests include theory and simulation of nonlinear metamaterials and photonic crystals, theory of optical solitons, and characterization of high performance optical switcher and optical filter for optical communications.
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Figure Captions
Figure 1 Schematic diagram of the proposed SPR biosensor by using few-layer Ti3C2Tx MXene to enhance the sensitivity.
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Figure 2 Variation of the reflectance with respect to the incident angle for (a) the conventional biosensor based on single Au film and (b) the proposed biosensor with few-layer Ti3C2Tx MXene.
Figure 3 Variation of (a) reflectance, (b) change in resonance angle (Δθ), (c) sensitivity and (d) sensitivity enhancement ((SL-S0)/S0) with respect to the different number of Ti3C2Tx MXene layers at λ=633 nm. Figure 4 (a-c) shows the change of sensitivity with the increase of Ti3C2Tx MXene layers at λ=633 nm for the
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proposed biosensors based on Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx, respectively; (d-f) shows the variation
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of the sensitivity enhancement with respect to the layers of Ti3C2Tx MXene at λ=633 nm for the proposed
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biosensors based on Ag-Ti3C2Tx, Al-Ti3C2Tx and Cu-Ti3C2Tx, respectively.
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Figure 5 (a) Variation of the reflectance with respect to the incident angle when the RI of sensing medium change from 1.33 to 1.36 (Δns=0.005) for the proposed Au-based biosensor with monolayer Ti3C2Tx MXene
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at λ=633 nm; (b) Movement of resonance angle (θr) for the proposed Au-Ti3C2Tx MXene-based SPR biosensor when the RI of sensing medium has an increase; (c) Variation of sensitivity with respect to the RI
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of sensing medium for the proposed Au-based biosensor with different number of Ti3C2Tx MXene layers at λ=633 nm; (d) The electric field distributions for the proposed SPR biosensor with monolayer Ti3C2Tx MXene
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when the RI of sensing medium has a change.
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Figure 6 (a) Variation of the reflectance with respect to the incident angle when the RI of sensing medium change from 1.325 to 1.335 (Δns=0.002) for the proposed Au-based biosensor with monolayer Ti3C2Tx MXene at λ=532 nm; (b) Movement of θr for the proposed Au-Ti3C2Tx MXene-based SPR biosensor when
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the RI of sensing medium has an increase and the θs is the resonance deviation angle; (c) Variation of Δθ with respect to the RI of sensing medium; (d) The electric field distributions for proposed Au-based biosensor with monolayer Ti3C2Tx MXene at λ=532 nm when the RI of sensing medium has a slight change.
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Table 1 RI of monolayer Ti3C2Tx MXene at visible range (nMXene=n+ik) [27].
633 nm
2.38
532 nm
2.64
k
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1.33 1.00
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Wavelength
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Table 2 The optimized layers, highest sensitivity and the sensitivity enhancement for the proposed SPR biosensor based on different metals and few-layer Ti3C2Tx hybrid nanostructures at λ=633 nm. Type of the
Sensitivity enhancement Highest sensitivity (°/RIU)
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Optimized Layers (L) biosensors
((SL-S0)/S0)
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Ag+Ti3C2Tx
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149
Al+Ti3C2Tx
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139
Cu+Ti3C2Tx
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147
16.8%
28.4% 46.3%
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