Angle-Scanning Surface Plasmon Resonance System with 3D Printed Components for Biorecognition Investigation

Surface plasmon resonance (SPR) is a real-time, label-free, and high-sensitive detection technology. SPR has been widely used in many applications such as biomolecular interaction analysis, environmental monitoring, and medical diagnostics. However, conventional SPR sensor systems usually require expensive equipment and complicated optics. In this paper, we have demonstrated a rapid prototyping of angle-scanning SPR for bioanalytical investigation. Rapid prototyping was attained by utilizing the FDM (fused deposition modeling) based 3D (three-dimensional) printing technology. Two rotating platforms were employed to drive the laser source and photodiode, respectively. A temperature regulation unit was incorporated to maintain the system temperature in order to reduce the temperature effect. The proposed SPR rapid prototyping yielded a refractive index resolution of 6.4×10 RIU (refractive index unit), and the biotin-avidin system validated the kinetics parameters measurement capability. The obtained results indicated that the FDM 3D printing has great potential for developing rapid-prototyping SPR system.


Introduction
Surface plasmon resonance (SPR) is a surface-sensitive optical technique that is used to study a thin layer on a metal surface.When P-polarized light is projected onto metal film, it can excite the oscillation of the electrons on the surface of the film.This oscillation is called surface plasmon wave (SPW).When the wave vector of the incident light matches the wave vector of SPW, the reflected light intensity decays rapidly, and the corresponding angle of the incident light is called resonance angle.Surface plasmon resonance (SPR) is a powerful analytical technology.It can detect the thickness of the films absorbed onto the sensor surface and interactions between biomolecules such as antigen-antibody or protein-DNA [1][2][3][4][5].In comparison with traditional detection methods, like X-ray, HPLC, and HPLC-MS, SPR technology can perform kinetics parameters (association and dissociation process) measurement, concentration measurement, and real-time molecule detection.It has been widely used in chemical, biology, agriculture, environment, and food safety [6][7][8][9][10][11][12].
Traditional SPR systems usually require expensive equipment and complicated optics.The cost of commercial SPRs varies from $10,000 to $5,000,000, and the refractive index resolution ranges from 10 −5 RIU (refractive index unit) to 10 −7 RIU.Many optical-fiber based SPRs have been studied intensively in the literature [13][14][15].The structure of these devices is relatively simple.However, the resolution and dynamic range are often sacrificed [16,17].To reach a high resolution of 10 −6 RIU or 10 −7 RIU, a spectrometer is always required for an optical-fiber SPR [18].
Compared with optical-fiber SPR, angle-scanning SPR has the capability of achieving high resolution and wide refractive index measurement range.A specific wavelength laser is irradiated on a metal film surface and scanned to achieve certain range of incident angle.However, anglescanning SPR requires more mechanical components, and 2 Advances in Condensed Matter Physics also the optical path is more complicated.Typical anglescanning SPRs are based on Kretschmann configuration.It involves many mechanical components manufacturing and complex optical path design.Therefore, constructing a compact home-made SPR is of great importance.
3D printing, also called additive manufacturing, since its origins in 1980s has been a tool for developing rapidprototyping products.With the expiration of 3D printing technology patents, this field has witnessed a great growth [19].FDM (fused deposition modeling) is one of the most popular 3D printing technologies.In FDM 3D printing, a heated nozzle is employed to melt raw material and extrude fused material onto a building platform, and it lays the fused material in layers.A wide range of materials can be printed, such as acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), high-impact polystyrene (HIPS), and thermoplastic polyurethane (TPU).After the emergence of open source RepRap project, desktop fused deposition modeling (FDM) 3D printers have become more popular among users, manufacturers, and researchers.It is simple, cost effective, and versatile.Application of FDM 3D printing technology to toys, prototypes, scientific tools, and medical equipment becomes more and more common [19][20][21][22][23].
Using 3D printing technology to design an SPR is an interesting and attractive thing.Recently, some studies have demonstrated the use of 3D printing in SPR platform.Hasan et al. developed a smartphone based SPR imaging platform for on-site biodetection [16].They utilized the 3D printing technique just to print the device holder.This system has a dynamic range less than 0.02 RIU, which limits the application for measuring large refractive index change.Bonyár et al. proposed an acrylic photopolymer material-based 3D printing to fabricate molds for PDMS (polydimethylsiloxane) casting and flow cell for SPR instrument.This method enabled the customization of SPR flow cell alone [24].However, 3D printing technology could be further applied to SPR.The use of 3D printing technology simplifies the process of manufacturing, accelerates the design of prototype, and makes designs more flexible.Furthermore, complex equipment, such as lathes and milling machines, is not important any more, even nonexpert users can quickly copy and build a 3D printed project.
In this study, we have demonstrated a rapid-prototyping angle-scanning SPR utilizing FDM 3D printing technology.PLA filament was chosen for its relative high tensile strength (∼50 MPa), good reproducibility, and low cost.With the help of 3D printing, the flow cell and optical path were designed to be modular and customizable.Refractive index resolution was found to be 6.4 × 10 −6 RIU.To validate the bioanalytical capability, biotin-avidin system was used as target analyte.
The developed SPR system includes a 650nm laser source (Shenzhen Fuzhe Technology Co. Ltd. 150mW, Shenzhen, China), a photodiode (2DU 10 mm × 10 mm), a polarizer (F-PZP-001 15 mm × 2mm), and a BK7 triangle prism (20mm × 20mm × 10mm).Au-coated glass slides (2mm × 8mm ×30mm) were prepared by coating 47.5 nm thickness of Au layer using sputtering deposition method.Prior to the deposition the Au film, the glass slide was precoated with 2 nm chromium layer to enhance the adhesion of the Au film.Cube 3 3D printer (3D Systems, USA) was used to print the necessary SPR system components (Fig S1).Silhouette Portrait (Silhouette America, Inc.) was used to cut the flow cell.Two rotating platforms (Beijing PDV Instrument Co., Ltd., China) were employed as the mechanical drivers.Open source boards (Arduino Mega 2560) accompanied with a self-made amplifier circuit comprise the hardware circuits.Two TECs (Thermoelectric Coolers) TES1-12704 (Hebei I.T. Co. Ltd., 3.3mm × 30mm × 30mm, Shanghai, China) were adapted as the temperature controller.
Schematic of rapid-prototyping angle-scanning SPR system block diagram is shown in Figure 1.Light from the laser source was first p-polarized by a polarizer and then projected onto the sensor surface via a triangle prism.The reflected light from the sensor surface was collected by a photodiode.The collected raw signal was amplified by a self-made amplifier circuit and sampled the built-in ADC (analog to digital converter) of the microcontroller unit ATmega2560 (Microchip Technology Inc., 8-bit AVR microcontroller, Chandler, Arizona, USA).Digital signals were eventually transmitted to the software on computer for analysis.The platform controller can drive the system to scan from 40 ∘ to 72 ∘ .When performing measurements, instead of scanning a whole SPR spectrum, an angle range of 3 ∘ around the resonance angle was scanned at a speed of 0.3 ∘ /s.For the temperature regulation unit, low temperature drift coefficient NTC (Negative Temperature Coefficient) resistors (SEMITEC Corporation, 103AT-4 Shape1, Japan) were employed to sense the system temperature and signals from NTC resistors were quantified by a 16-bit high-precision ADC ADS1115 (Texas Instruments Inc. USA).A microcontroller ATmega328 (Microchip Technology Inc., 8-bit AVR microcontroller, Chandler, Arizona, USA) was used to obtain the temperature information from ADS1115 via IIC (Inter-Integrated Circuit) communication protocol and control the heating or cooling of the flow cell through TECs.A PID (proportional-integral-derivative) controller algorithm was incorporated with ATmega328 to maintain a stable temperature circumstance.
The flow cell module consists of rubber flow cell and stainless part.Arrangement of the flow cell module is shown in Fig. S2.The rubber flow cell of volume 65 L and dimension 5.3 mm × 23 mm × 0.5 mm was designed and constructed  using desktop cutter Silhouette Portrait.However, one can customize the shape of flow cell by modifying the digital design file and snip it in the rubber film with Silhouette Portrait.
Overview of the developed rapid-prototyping SPR system was depicted in Figure 2. The system was constructed with 3D printed components.Cube 3 desktop version 3D printer was used to print the components with PLA filaments.The component models were designed using SolidWorks 2012 software and saved as STL (Standard Triangle Language) file format.Then, these STL files were imported into the 3D printer's software Cubify version 3.9.0(3D systems, USA).Two rotating platforms were employed and a 3D printed optical platform was installed onto the rotating platforms.The laser source and photodiode can be easily mounted onto the optical platform with the 3D printed holders.Unlike the laser and photodiode, the flow cell module was removable and customizable.Furthermore, the alignment of optical parts can be achieved effortlessly.
The SPR sensor functionalization process is shown in Figure 3.To functionalize the SPR sensor with biotin, Aucoated glass slide was first cleaned ultrasonically with 4% sodium dodecyl sulfate (SDS) solution and rinsed with deionized water.Then it was exposed to UV light for 20 minutes.Au-coated glass slide was then immersed in 10 mM cysteamine solution for 2 hours and subsequently rinsed with deionized water to remove the residual cysteamine.The resulting sensor was then immersed in 0.2 mg/mL biotin solution containing 15 mM EDC and 5 mM sulfo-NHS.Then the sensor was rinsed in deionized water and absolute ethanol and dried under a stream of nitrogen for further use.

Experimental
We used Tianmin contact angle meter to measure water contact angle at each modification stage.SPR sensor chips were mounted on the plane of contact angle meter.4 L of deionized water was dropped onto the sensor chip surface.The droplet cross-section was recorded as image file by the camera on the contact angle meter.Open source software ImageJ was used to estimate the contact angle from recorded image files.
The most benefit of SPR biosensor technology is the determination of kinetics of biomolecular interactions.It can be determined that the interaction of analytic A and ligand B is immobilized on sensor surface.It usually uses the simplest case A + B → AB.In this study, The kinetic parameters measurement capability of the developed system was validated using the biotin-avidin system.Affinity interactions between biotin and avidin were characterized by the association rate constant k  , dissociation rate constant k  , and equilibrium association constant K  , where A is the injected analyte, B is the immobilized ligand, and AB is the analyte-ligand complex.The experiment data was fitted with A + B → AB interaction model.The interaction process can be described as (1).The response signal R of SPR system is proportional to the amount of [AB], k  is the apparent reaction rate constant, t is the interaction time, and the maximum response signal  max is proportional to the initial [B].A series of concentrations of [A] solutions were used to derive the relationship between k  and different concentrations of [A], and a regression curve was obtained from (2).Thus k  is the slope of the regression, k  is the intercept of the regression curve, and For fast kinetics parameters measurement, affinity interactions between biotin and avidin were characterized by the association rate constant k  , the dissociation rate constant k  , and the equilibrium association constant K  .However, the interaction process is divided into association phase, equilibrium phase, and dissociation phase.The dissociation rate constant k  was obtained from the dissociation phase by (3).Then it was substituted into (1) to obtain k  from the association phase.Thus K  was computed as the ratio of association and dissociation rate constants.

Results and Discussion
Prior to conducting the biomedical experiment, effectiveness of the sensor surface modification should be verified.Contact angle measurement is a powerful tool to examine every step during the sensor functionalization.Results of contact angle measurement are shown in Figure 4. Bare Au surface exhibited a value of 88.378 ∘ .After 20 min of UV exposure, the contact angle decreased to 71.922 ∘ .This contact decrease indicates that the UV exposure could effectively remove the impurities on the Au film.When the cysteamine self-assembled monolayers (SAMs) formed on Au film, the contact angle became 46.720 ∘ , which indicated that the cysteamine SAM is terminated with a hydrophilic domain −NH 2 [25].Subsequently, after the immobilization of biotin onto the cysteamine SAM, the contact angle decreased to 21.911 ∘ from 46.720 ∘ of the cysteamine SAM.The reason was that the ureido-and tetrahydrothiophene-rings of biotin are more hydrophilic than −NH 2 terminated cysteamine SAM [26].The contact angle of biotin SAM was smaller than the previously reported one in study [27], and this may be due to the well-organized biotin SAM.Thus, the above results showed that the SPR sensor chip was prepared successfully.
The interactions between biotin and different concentrations of avidin solution are plotted in Figure 5.A higher concentration of avidin showed a faster association rate.Biotin SAM on the sensor surface was saturated in a short time.Meanwhile, at lower concentration of avidin, the association process occurred slowly, and it took a longer time to reach equilibrium.After the saturation of biotin on the sensor surface, the binding of biotin and avidin reaches equilibrium, and the SPR response signal reaches  max .The relationship between k  and various concentrations of avidin was plotted as shown in Figure 6.The linear regression equation was obtained as k  = 4.9×10 4 ×[C] + 5.4×10 −5 , where C is the avidin concentration.Thus the kinetic parameter k  was first obtained as 5.4×10 −5 s −1 , k  was obtained as 4.9×10 4 M −1 s −1 , and the K  was computed as 9.8 × 10 8 M −1 , which is close to the K  reported by Zhao et al. [28].
For fast kinetic parameters measurement, 73.2 nM avidin was used.The interaction process during the association phase, equilibrium phase, and dissociation phase was recorded as in Figure 7.For the dissociation phase, k  was derived as 0.0012 s −1 .After substituting k  into (1), we obtain k  = 3.551 × 10 4 M −1 s −1 and K  = k  /k  = 2.96 × 10 7 M −1 .The kinetics parameters obtained using the two above methods were different.However, the fast kinetics parameters measurement utilizes the association and dissociation phase of the interaction process at only one concentration.Comparing these two methods, the results showed that the system has the capability to analyse the interaction between protein and ligands by different analytic concentrations.

Conclusions
This work presented the development of a rapid-prototyping surface plasmon resonance utilizing FDM 3D printing  technology.The developed device was modular and the optical path and flow cell were customizable.The system performance was validated with a series of experiments, and our system showed a high resolution of 6.4 × 10 −6 RIU.Furthermore, kinetics parameters measurement capability was validated by biotin-avidin system.In the future, we will aim to develop our 3D-SPR to be more modular and easier to construct, so that many institutes or researchers can build their own 3D-SPR.

Figure 1 :
Figure 1: Schematic representation of the complete SPR system.

Figure 2 :
Figure 2: Overview of the designed SPR system by rapid-prototyping 3D printing.

Figure 5 :
Figure 5: Association curve of biotin and avidin.

Figure 6 :
Figure 6: k app versus different concentrations of avidin.