Targeted Sub-Attomole Cancer Biomarker Detection Based on Phase Singularity 2D Nanomaterial-Enhanced Plasmonic Biosensor

Highlights A zero-reflection-induced phase singularity is achieved through precisely controlling the resonance characteristics using two-dimensional nanomaterials. An atomically thin nano-layer having a high absorption coefficient is exploited to enhance the zero-reflection dip, which has led to the subsequent phase singularity and thus a giant lateral position shift. We have improved the detection limit of low molecular weight molecules by more than three orders of magnitude compared to current state-of-art nanomaterial-enhanced plasmonic sensors. Abstract Detection of small cancer biomarkers with low molecular weight and a low concentration range has always been challenging yet urgent in many clinical applications such as diagnosing early-stage cancer, monitoring treatment and detecting relapse. Here, a highly enhanced plasmonic biosensor that can overcome this challenge is developed using atomically thin two-dimensional phase change nanomaterial. By precisely engineering the configuration with atomically thin materials, the phase singularity has been successfully achieved with a significantly enhanced lateral position shift effect. Based on our knowledge, it is the first experimental demonstration of a lateral position signal change > 340 μm at a sensing interface from all optical techniques. With this enhanced plasmonic effect, the detection limit has been experimentally demonstrated to be 10–15 mol L−1 for TNF-α cancer marker, which has been found in various human diseases including inflammatory diseases and different kinds of cancer. The as-reported novel integration of atomically thin Ge2Sb2Te5 with plasmonic substrate, which results in a phase singularity and thus a giant lateral position shift, enables the detection of cancer markers with low molecular weight at femtomolar level. These results will definitely hold promising potential in biomedical application and clinical diagnostics. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00613-7.


S1.1 Relationship between lateral position shift and reflectivity
In our simulation analysis, we calculated the reflection coefficient and lateral position shift under different incident angle using both Au-only substrate and atomically thin GST-on-Au substrate. To demonstrate the relationship between the reflectivity and lateral position shift more clearly, a plot showing their negative correlation was drawn through Fresnel equations and transfer matrix method (TMM) and calculated with a MATLAB programming ( Figure S1). This explains more explicitly that the maximum lateral position shift is achieved at the minimum reflectivity. Therefore, it is very essential to enhance the zeroreflection effects of the sensing substrate. Log Reflectivity (Arb.units)

S1.2 Finite element analysis (FEA)
We use finite element analysis (FEA) (COMSOL Multiphysics 5.2) to study the electric field distribution on this 2D GST-on-Au sensing substrate at the resonance angle. As shown in Figure S2 (a), a large electric field enhancement occurs at the sensing interface when surface plasmon resonance is excited. The resonance also results in a minimum reflectance and an enhanced lateral position shift in the reflected beam. We have conducted a comparison between the reflectivity and the lateral position shift of the Au-only substrate and our 2D GST-on-Au nanomaterial based substrate in Figure S2 (b)(c). In both cases, the largest lateral position shift coincides with the minimum reflectivity point. An importance outcome of our analysis is that the addition of atomically thin GST material leads to a much deeper resonance dip. The corresponding maximum lateral position shift is 2107.33 μm, which is nearly 100 times larger than that associated with the case of using Au-only substrate. We can therefore assert that the atomically thin 2D GST layer will offer a superior sensitivity enhancement.

S2.1 Optical characterization
The dielectric constant of GST in relation to the photon energy was measured through spectroscopic ellipsometry. For our experimental configuration, which uses a He-Ne laser (632.8 nm), the dielectric constant was determined to be 13.00+11.10i.

S2.2.1 Angular scanning reflectivity spectra in air
To evaluate the performance of the atomically thin GST-on-Au sensing substrate, we first measured its angular scanning reflectivity spectra in air. As a comparison, we also measured the Au-only substrate. The experimental results show good agreement with theoretical calculations, which confirms the reliability of our device and serves as a good calibration for the assessment of sensing performance. As shown in the Figure S4, the presence of GST material clearly leads to a deeper resonance dip (minimum intensity lowered by 50%).

S2.2.2 Standard sensor evaluation using glycerol solutions
In

S2.2.3 Biomolecule (BSA) sensing performance
The real-time biosensing capability of our device is also demonstrated through monitoring binding of BSA biomolecules, which have relatively high molecular weight (66463 Da), at different concentration levels. Solutions of the biomolecules with different concentrations ranging from 10 fм to 10 μм were detected and recorded in Figure S6 based on lateral position shift measurement, which shows a linear increase in lateral position shifts with increasing BSA concentrations. Signal saturation will start when the concentration goes above Figure S6. Detection of BSA molecules based on lateral position shift.

S2.2.4 Detection of non-specific binding
To demonstrate the specificity of our sensing device, the non-specific binding between TNF-alpha and BSA molecules has also been detected in comparison with the specific antibody-antigen binding. We carried out TNF-α (tumor necrosis factor α) antigen detection using a sandwich immunoassay strategy. After flowing antigen containing solutions to the sensing substrate coated with antibody solutions, we further injected the capture antibody -monoclonal anti-TNF antibody to the sensing substrate.
The lateral position shift signal can be increased to 16.70 μm when flowing 10 pм antibodies. As a negative control experiment, we use BSA as the control antibody [1][2][3][4]. As shown in Figure S7, the lateral position shift signal change when flowing BSA with a large concentration (10 5 times higher than anti-TNF antibody) is much smaller compared to flowing antibody, which shows the high specificity of our sensing device. Figure S7. Detection of specific and non-specific binding based on lateral position shift. Blue curve shows the signal change when flowing capture antibody -monoclonal anti-TNF antibody to the sensing substrate while the black curve shows the signal change of flowing BSA as control antibody.

S2.2.5 Cancer marker detection
The differential lateral position shift signals acquired during TNF-α detection with replicate measurements were summarized. The lateral position shift signal can reach 6.52 μm for 1 fм cancer marker and 53.15 μm for 1 nм cancer marker using GST-on-Au substrate. Figure S8. Detection of TNF-α based on lateral position shift.