Label-Free Sandwich Imaging Ellipsometry Immunosensor for. Serological Detection of Procalcitonin

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1 Supporting Information Label-Free Sandwich Imaging Ellipsometry Immunosensor for Serological Detection of Procalcitonin Yike Li, Wei Liu,, Gang Jin,, Yu Niu *,, Yiping Chen *, ǁ, Mengxia Xie *, Analytical and Testing Center of Beijing Normal University, Beijing , China Institute of Microelectronics, Tsinghua University, Beijing , China NML, Beijing Key Laboratory of Engineered Construction and Mechanobiology, Institute of Mechanics, Chinese Academy of Sciences, Beijing , China School of Engineering Science, University of Chinese Academy of Science, Beijing , China ǁ CAS Key Laboratory for Biological Effects of Nanomaterials and Nanosafety, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing , China Corresponding Author Yu Niu, Phone: (86) Yiping Chen, Phone: (86) Mengxia Xie, Phone: (86) S-1

2 Table of Contents Introduction and detection principle of the IE immunosensor S-3 Deduction of the relationship between the IE immunosensor signal and the analyte concentration S-5 Optimization of the azimuth angles of the IE immunosensor S-9 Screening cab with high binding affinity to PCT S-9 Optimization of the cab concentration in the SSIE immunosensor S-11 Optimization of the dab detection time S-12 Determining the limit of detection S-13 Comparison between the IE immunosensor (using I sandwich as the detection signal) and the commercial ELECSYS PCT assay S-14 S-2

3 Introduction and detection principle of the IE immunosensor Ellipsometry is a conventional optical technique for measuring changes in light polarization state, and it can determine film thickness with a resolution of 0.1 nm or better as a nondestructive and label-free method. Imaging ellipsometry (IE) is an enhancement of standard single beam ellipsometry that combines the power of ellipsometry with microscopy. High spatial resolutions with the vertical and lateral resolutions at the magnitude of 0.1 nanometer and 1 micron, respectively, can be achieved with a large view area around several square centimeters, which allows multisensing for target microarrays simultaneously. IE used in our lab is in a typical polarizer-compensator-sample-analyzer (PCSA) arrangement and performed under off-null condition. When it is used to detect the information about the sample, the reflection intensity is recorded from the detector with the polarizer, compensator, and analyzer at fixed angular positions. The reflection intensity is highly independent with the polarizer, the compensator, the analyzer, the light source, and the detector. By such a setup of the IE, the sampling process doesn t require the components and devices of the ellipsometer to be adjusted in a mechanical way as employed in other ellipsometry techniques, allowing it suitable for high-speed measurement. The concept of IE immunosensor was first proposed in 1995 for the visualization of biomolecule interactions and then the commercial products based on single wavelength and spectroscopic IE made by Nanofilm Company ( Gedig et al., 2008) have come out on the market. So far, many significant works, such as the off-null sampling method, spectroscopic technique, automatic control, microarray surface modification, microfluidic reactor, biosystem optimization and protein interaction database, are introduced into IE immunosensor to improve its performance and feasibility. IE is used to characterize and visualize the surface mass density distribution of the protein layers attached on a patterned surface on a solid substrate layer in grayscale. A ligand and its receptor could assemble into biocomplexes due to their affinity. When the receptor in the solution interacts with its corresponding ligand, they assemble into complex upon their affinity. The surface S-3

mass density where the interaction takes place becomes higher. A significant increase of the surface mass density indicates that the solution contains the receptor against the ligand.

4 mass density where the interaction takes place becomes higher. A significant increase of the surface mass density indicates that the solution contains the receptor against the ligand. With the visualization of IE, the increase could be determined, and in this way, the existence of the receptor in the solution could be verified quantitively. The general detection procedure with the IE immunosensor can be summarized as the following successive steps. First, the substrate is modified with chemical reagents to form a chemically modified surface. Then, the modified surface is placed in the microfluidic reactor system to form patterned cells in an array format. Different ligand solutions are separately pumped into each cell to immobilize ligand in the microarray. After that, the analyte solution is delivered to react with ligands. Finally, the microarray is sampled by the IE reader to determine molecular interaction results. Figure S1. The principle of the IE immunosensor to detect protein interactions. S-4

5 Deduction of the relationship between the IE immunosensor signal and the analyte concentration For the direct IE immunosensor, the ligand immobilized on the biosensing substrate recognizes the analyte in the sample solution to form ligand-analyte complex. The recognition process is given by ligand + analyte k a k d ligand analyte where k a and k d are the association rate constant and the dissociation rate constant, respectively, and thus the dissociation equilibrium constant K D = k d /k a. Under a pseudo-first-order interaction assumption, the rate equation of ligand-analyte complex formation governing the time evolution is given by dc ligand analyte /dt = k a c ligand c analyte k d c ligand analyte (S1) where c analyte, c ligand, and c ligand analyte are the concentrations of the analyte in solution, the ligand and ligand-analyte complex on the biosensing substrate, respectively. During the recognition process, the analyte solution is delivered to the biosensing substrate constantly. The analyte concentration can be regarded as a constant. Assuming (γ ligand ) 0 stands for the initial amount of the ligand before analyte solution is delivered to the biosensing substrate, eq S1 can be expressed by dc ligand analyte /dt = k a ((γ ligand ) 0 c ligand analyte ) c analyte k d c ligand analyte (S2) Doing an integral over time in eq S2, we will get c ligand analyte = (γ ligand) 0 c analyte (1 e (k a c analyte +k d ) t ) K D + c analyte (S3) At the equilibrium of the recognition process, lim t e (k a c analyte +k d ) t = 0, c ligand analyte S-5

6 is given by c ligand analyte = (γ ligand) 0 c analyte K D + c analyte (S4) For the sandwich strategy, where the ligand is namely the capture antibody (cab), cab on the substrate first captures the analyte to generate cab-analyte complex, the process is given by cab + analyte k a k d cab analyte Then cab-analyte complex on the substrate interacts with the detection antibody (dab) in the solution to form cab-analyte-dab complex, the recognition is given by cab analyte + dab k a cab analyte dab k d With the same assumption and deduction like the recognition process in the direct strategy, the amount of cab-analyte-dab complex at the equilibrium can be given by c cab analyte dab = (γ cab ) 0 c dab K D + c c analyte dab K D + c analyte (S5) where c dab and K D are the dab concentration in solution and the dissociation equilibrium constant of the second step, respectively. As the IE immunosensor uses the conventional polarizer-compensator-sample-analyzer configuration in Scheme 1, the IE immunosensor signal, δi, can be given by δi = I 0 ( δr s R s + α 1 δψ + α 2 δδ) (S6) where I 0 is the detected light intensity for the bare substrate and R s is the complex reflectance for s-polarized light which is given by R s = R s 2. δψ and δδ are the ellipsometry parameter variations, while α 1 and α 2 are the coefficients for δψ and δδ, respectively. S-6

7 Near the pseudo-brewster angle of the substrate, R s is insensitive to the film thickness change on the surface, which implies δr s 0. For protein layer, its film thickness is proportionate to its surface mass density. When the protein adsorbs on the substrate, the relationship between the surface mass density of the protein layer, δγ, and the ellipsometry parameter variations, δψ and δδ, can be given by According to eqs S6 and S7, we have δψ δγ δδ δγ (S7) I = k Γ Γ = k Γ n M S (S8) where k Γ, n, M, and S are the proportional coefficient, the analyte amount, analyte molecular weight and the substrate area. The IE immunosensor signal of a complex protein layer is the sum of each protein layer on the substrate, which is given by I = K n M (S9) where K = k Γ /S. For the direct IE immunosensor, after the addition of the analyte, the IE immunosensor signal change caused by the analyte, I analyte, is given by Equation S10 can be expressed as I analyte = K (γ ligand) 0 c analyte K D + c analyte M analyte (S10) I analyte = A c analyte B + c analyte (S11) where A and B equals to K (γ ligand ) 0 M analyte and K D, respectively For the sandwich strategy, after the addition of dab solution, the IE immunosensor signal change caused by forming cab-analyte-dab complex, I dab, is given by S-7

8 I dab = K Equation S12 can be expressed as (γ cab ) 0 c dab K D + c c analyte dab M K D + c dab (S12) analyte I dab = A c analyte B + c analyte (S13) where A and B equals to K (γ cab) 0 c dab M dab K D +c dab and K D, respectively. S-8

9 Optimization of experimental conditions Optimization of the azimuth angles of the IE immunosensor To improve the IE immunosensor detection performance, the azimuth angles of the polarizer and the analyzer were optimized. The optimization is crossly performed by simultaneously rotating the polarizer and the analyzer with the interval of 0.1 to compare the IE immunosensor signal under different conditions. When the azimuth angles of the polarizer and the analyzer were optimized to 10.2 and 3.3, respectively, the effective signal of the IE immunosensor reaches the maximum. Screening cab with high binding affinity to PCT According to eq S10, higher binding affinity between cab and PCT helps to improve the IE immunosensor signal. In order to screen an eligible cab, seven different commercial antibodies from Genstars (Beijing, China), Abcam (Cambridge, U.K.), Invitrogen (Massachusetts, U.S.A.), Novus (Littleton, U.S.A.), R&D (Abingdon, U.K.), and Biovision (San Francisco, U.S.A.) have been tested by the direct IE immunosensor to evaluate their binding affinity towards PCT. The detection of 32 ng ml -1 PCT in the direct IE immunosensor has been carried out with each kind of antibody at different concentrations (50 μg ml -1, 100 μg ml -1, 200 μg ml -1 ). The results of 100 μg ml -1 antibodies are shown in Table S1 and Figure S2. Among these antibodies, the antibody from Genstars brings the highest signal value of the IE immunosensor, indicating a preferable bioactivity after immobilizing on the substrate and a higher binding affinity towards PCT. Thus, the antibody from Genstars is selected as the cab in the SSIE immunosensor for further study. S-9

10 Table S1. The grayscales for the detection of PCT using different commercial antibodies as cab in the direct IE immunosensor Antibodies Biovision Abcam Invitrogen Genstars R&D The direct IE Novus 44d9 Novus 18B7 immunosensor signal 1.13± ± ± ± ± ± ±0.07 (Grayscale) Figure S2. The detection of PCT using different commercial antibodies as cab in the direct IE immunosensor. S-10

11 Optimization of the cab concentration in the SSIE immunosensor PCT at the concentrations of 2 and 32 ng ml -1 as well as 200 μg ml -1 dab have been chosen to optimize the surface amount density of cab. We have compared the IE immunosensor behavior between different cab concentrations ranging from 2 to 200 μg ml - 1. Figure S3 demonstrates the variance of the IE immunosensor signal towards PCT concentrations. For PCT concentrations of 2 and 32 ng ml -1, the growth of immobilization capacity arrives saturation at 100 μg ml -1 upward, before which the growth increases progressively. Yet the binding capacity decreases with continuous increment of cab due to the steric hindrance on the IE immunosensor substrate. Therefore, 100 μg ml -1 is optimized for cab concentration. Figure S3. Concentration optimization of cab from 2 to 200 μg ml -1. S-11

12 Optimization of the dab detection time To achieve fast-speed detection, the detection time of dab is studied. The same to the eq S3, larger amount of dab helps to decrease the detection time of dab. dab concentrations from 50 to 400 μg ml -1 were chosen to detect PCT of 32 ng ml -1. Figure S4 shows the detection time for the different amounts of dab. For the dab concentrations from 50 to 200 μg ml -1, the detection time decreases with the increasing dab amount. When further increasing the dab concentration, the detection time remains at 3 min. So the detection time is 3 min with dab concentration at 200 μg ml -1. Figure S4. The detection time for the different concentrations of dab. S-12

13 Determining the limit of detection The calibration curves of the direct IE immunosensor and the SSIE immunosensor are fitted with the eqs S11 and S13, respectively. Their LODs are given by B 3σ LOD direct = A 3σ LOD sandwich = B 3σ A 3σ where σ is the standard deviation of 20 independent measurements of the blank control. (S14) S-13

14 Comparison between the IE immunosensor (using I sandwich as the detection signal) and the commercial ELECSYS PCT assay Figure S5. Comparison between the IE immunosensor and the commercial ELECSYS PCT assay for PCT detection in 12 human serum samples. I sandwich is taking as the detection signal for the IE immunosensor. S-14

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