Self-referencing SPR biosensing with an ultralow limit-of-detection using long-wavelength excitation

https://doi.org/10.1016/j.snb.2020.128935Get rights and content

Highlights

  • Surface plasmons at long-wavelength band can achieve a larger decay length and higher sensitivity.

  • A self-referencing sensing strategy is employed to eliminate cross-sensitivity problem and further improves accuracy.

  • An ultralow LOD were recorded by using graphene oxide-assisted Au nanoparticle conjugates as signal amplification tags.

Abstract

There is an urgent need of label-free technique with low limit of detection (LOD) for biomedical research and early clinical diagnostics. Surface plasmon resonance (SPR) as a label-free approach has been exploited in DNA determination, but the LOD of most SPR sensors is inferior to labeling approaches. A novel approach for SPR biosensing with low LOD was reported in the present study. SPR for long wavelength excitation is theoretically and experimentally demonstrated to possess a high refractive index sensitivity of 11773.93 nm/RIU due to the long decay length of the electromagnetic field near the metal surface, which is optimized by an order of magnitude compared to traditional SPR sensor. Subsequently, self-referencing sensing strategy was employed to eliminate cross-sensitivity problem and further improve sensing accuracy significantly. Finally, we focused on using this sensor to measure DNA hybridization events using graphene oxide-assisted Au nanoparticle conjugates as signal amplification tags. It achieves a large dynamic range of 10−15-10-11 M and a low LOD of 0.2 fM for target DNA determination. The proposed SPR sensing architecture is technologically simple, highly sensitive and extensively appropriate for existing SPR instrumentation, so it has a great potential for low LOD of various biomolecules.

Introduction

DNA sequence determination is important in many fields, such as genetics, medical diagnostics and criminal investigation [1,2]. Labeling of target molecules is main DNA determination approach [3], but it is high in cost, complex in operation and unfit for on-site or real-time detection. Surface plasmons confined to a metal/dielectric interface are widely known to be sensitive to the refractive index (RI) of the dielectric within the electromagnetic field near sensing surface [4]. Surface plasmon resonance (SPR) spectroscopy has developed rapidly due to their advantages of high sensitivity and real-time response. SPR technique is ideally suited to detect surface bioaffinity adsorption of various biological analytes, so it has been used for DNA determination over past decades [5,6]. However, the limit of detection (LOD) of most SPR sensors is inferior by two orders of magnitude to labeling approaches [[7], [8], [9]], which is one of main barriers for further development in SPR technique.

Plenty of approaches have been employed to improve LOD of SPR biosensor, mainly focusing on sensitivity enhancement, accuracy improvement and signal amplification. Exciting long range SPR model [10] or adding dielectric film with high permittivity [11] can be used to achieve high sensitivity by enhancing evanescent field. But due to the sensing structures of two approaches, biomolecular modification is limited and it prevents them from being applied to biosensing. Using phase modulation [12] or modifying the prism RI [13] can improve sensitivity, while resulting in lower dynamic detecting range. Due to their strong local field enhancement, nanomaterials [14] and nanostructures [15] as SPR sensing films can also improve surface sensitivity. Nevertheless, complex fabrication process and chemical synthesis go against sensing applications. Using a larger excitation wavelength can excite a greater decay length [16], which is rarely reported in sensing applications as a sensitivity enhancement approach. Therefore, it is expected to achieve lower LOD and provide new opportunities for biosensing purposes.

Besides of sensitivity optimization, SPR sensor can improve LOD by increasing the accuracy. Because all RI variations within decay region can result in a change in output signal, specific response signal is weak and easily masked by undesired signals (due to bulk RI variation, temperature fluctuation or nonspecific adsorption) for detecting low-concentration sample. Self-referencing SPR sensors are generally used to solve this problem by adding an extra channel, which commonly utilizes dual-mode spectroscopy [17,18] or dual-wavelength spectroscopy [19,20]. The referencing channel can effectively overcome the cross- sensitivity problem and improve sensing accuracy.

In addition, another very desirable approach that can be used to enhance LOD is a combination with amplification approach. Metallic nanoparticles [21], magnetic nanoparticles [22], quantum dots [23] and carbon nanomaterials [24] can be used as amplification materials due to their good performance such as low cost, high stability and strong tunability. Among them, graphene material has become one of the hottest topics in many fields due to its large surface area, excellent electrical conductivity and biocompatibility [25]. For example, graphene oxide (GO) has been used to fabricate nanocomposite with metal nanoparticles, which displays surface-enhanced Raman scattering [26] and enhances sensing sensitivity [27].

In this paper, we designed a novel SPR sensor by employing long-wavelength (LW) excitation, self-referencing chip and GO-Au nanoparticle (AuNP) conjugates in combination. SPR for LW excitation on gold film was used to replace traditional SPR system and theoretically achieved greater decay length (∼1000 nm) and high bulk sensitivity (∼15,000 nm/RIU). LW-SPR is more than 5 times higher than conventional SPR, both in terms of decay length and bulk sensitivity. Self-referencing approach was employed to eliminate cross-sensitivity problem and improve sensing accuracy. And GO-AuNP conjugates were synthesized and used as signal amplification tags to form sandwich complex on the sensing surface. It is worth noting that, DNA sample is successfully detected with a wide dynamic range of 10−15-10-11 M and ultralow LOD is estimated to fM level. The designed SPR sensing approach has a great potential for ultralow LOD of various biomolecules. The label-free and highly sensitive SPR technique enables the real-time interrogation of small molecules, allowing for detection of low concentration.

Section snippets

Reagents

1-(3-Dimethylamino-propyl)-3-ethylcarbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS) and sodium citrate (99 %) were purchased from Shanghai Saen Chemical Technology Co. Ltd.; Tris-(2-carboxyethyl)-phosphine hydrochloride (TCEP), phosphate buffer saline (PBS, NaCl 136.89 mM; KCl 2.67 mM; Na2HPO4 8.1 mM; KH2PO4 1.76 mM, pH 7.4) and ultrapure water (18.2 MΩ cm) were purchased from Shanghai Sheng Gong Co. Ltd.; GO dispersion solution (1 mg/mL) was purchased Jiangsu XFNANO Materials Tech

LW-SPR sensing character

Compared with traditional SPR sensing system, our operating wavelength locates in LW band by changing illumination condition at a smaller incident angle. For attenuated total reflection (ATR) method based on the prism coupler, the incident light reflected at the prism-metal interface evanescently penetrates through the metal layer and excites an SPW at the outer boundary of the metal layer. For researching the LW-SPR characteristic, we numerically calculated the permittivity of the metal layer,

Conclusion

In summary, a novel self-referencing LW-SPR biosensing system for DNA determination was proposed, constructed and demonstrated. Thanks to greater decay length of SPW, highly sensitive characteristics of SPR sensor for LW excitation were analyzed and validated. Self-referencing measurement was applied to further improve accuracy by eliminating influences from bulk RI variation and nonspecific adsorption. This approach provides enhanced RI sensitivity (11773.93 nm/RIU), which is approximately

CRediT authorship contribution statement

Shimeng Chen: Conceptualization, Methodology, Software, Validation, Investigation, Writing - original draft, Visualization. Yun Liu: Data curation, Writing - review & editing, Supervision, Investigation. Qingxu Yu: Funding acquisition. Wei Peng: Resources, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge funding from National Natural Science Foundation of China (Grant No. 61727816, 61705031, 61520106013) and China Postdoctoral Science Foundation (Grant No. 2017M610175 and 2018T110216).

Shimeng Chen received a B.S. degree in Dalian University of Technology Dalian, China, in 2014. She is currently pursuing her Ph. D degree in Dalian University of Technology, Dalian, China. Her research interests include biochemical fiber sensors and surface plasmon resonance biosensors.

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      Citation Excerpt :

      It is known that the use of larger excitation wavelength in SPR can improve the decay length of SPP wave [26]. In a recent work by Chen et al., it was also showed that the use of higher wavelength excitation can be utilized for enhancing the detection sensitivity [27]. In the wavelength modulation based SPR setup, where a broadband source is used, it is easier to modulate the surface plasmon resonance wavelength towards infra-red by slight modulation of the incidence angle.

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    Shimeng Chen received a B.S. degree in Dalian University of Technology Dalian, China, in 2014. She is currently pursuing her Ph. D degree in Dalian University of Technology, Dalian, China. Her research interests include biochemical fiber sensors and surface plasmon resonance biosensors.

    Yun Liu received a Ph.D degrees in Dalian University of Technology Dalian, China, in 2016. He has been a Lecturer with the school of physics, Dalian University of Technology. His research interests include optical fiber sensor, surface plasmon resonance sensor.

    Qingxu Yu received Ph. D. degree in Dalian University of Technology Dalian, China, in 1990. Then he has been working at Dalian University of Technology, he is currently a professor in School of Optoelectronic Engineering and Instrumentation Science. His research interests include optical fiber sensor and optical fiber communication.

    Wei Peng received Ph. D. degree in Dalian University of Technology, China in 1999. Then she joined Center for Photonics Technology within Department of Electrical Engineering at Virginia Polytechnic Institute and State University as worked as a Postdoctoral Associate. From 2004–2007, she worked as a research assistant professor at Department of Chemistry and Biochemistry, Arizona State University. During 2007–2009, she worked as a Senior Research Scientist in Applied Technologies Division at Physical Optics Corporation, Inc. She took a professor faculty position in College of Physics and Optoelectronic Engineering at Dalian University of Technology. Her research interests include subwavelength optics and surface plasmon resonance.

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