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Abstract
Chiral plasmonic nanostructures can generate large superchiral near fields owing to their intrinsic chirality, leveraging applications for molecule chirality sensing. However, the large structural chirality of chiral nanostructures poses the risk of overshadowing molecular chiral signals, hampering the practical application of chiral nanostructures. Herein, we propose an achiral nanorod that shows no structural chirality and presents strong superchiral near-fields with linearly polarized incidence. The mechanism of the strong superchiral near-field originates from the coupling between the evanescent fields of the localized surface plasmon resonance and incident light. The enhanced near-field optical chirality at the corners of the nanorods reached 25 at a wavelength of 790 nm. Meanwhile, the sign of optical chirality can be tuned by the polarization of the incident light, which provides a convenient way to control the handedness of the light. Furthermore, the enantiomers of D- and L-phenylalanine molecules were experimentally characterized using an achiral platform, which demonstrated a promising nanophotonic platform for chiral biomedical sensing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Geometrical chirality is defined as an object that cannot overlap with its mirror image through translations and rotations [1,2]. Chiral biomolecules such as sugars, amino acids, DNA, and enzymes are found everywhere in the human body, which determine the healing potency and toxicity of chiral drugs [3,4]. To discriminate the chirality of chiral biomolecules, circular dichroism (CD) spectrometry is widely employed for enantiomeric differentiation in the ultraviolet (UV) frequency range owing to the different absorbances of chiral molecules interacting with right- and left-handed circularly polarized light. However, the size mismatch between the chiral molecules (a few tens of nanometers) and wavelength of incident light (>150 nm) leads to weak biomolecule chiroptical signals [5,6]. Therefore, the accurate identification of chiral analytes using a CD spectrometer remains a challenging task that requires high-concentration solutions of chiral analytes and a long integral time for chirality detection.
Recently, chiral plasmonic nanostructures that can generate strong and uniform superchiral near fields have been widely applied for high-sensitivity chiral sensing [7–11]. Enhanced chirality sensing by superchiral near-field can probe the secondary structural information of chiral biomolecules [12]. Here, the superchiral near fields can be quantified by the optical chirality defined as $- {\varepsilon _0}\omega {\mathop{\rm Im}\nolimits} ({E^\ast } \cdot B)/2$ [13], where ω, ɛ0, E and B are the angular frequency, vacuum permittivity, electric field, and magnetic field, respectively. The optical chirality can be used as the figure of merit (FOM) to characterize the sensor [14]. According to the definition of optical chirality, chiral plasmonic nanostructures show stronger optical chirality than achiral plasmonic nanostructures, making them competitive nanophotonic platforms for chiral sensing [11,15]. However, because of the strong background CD from the geometric chirality of chiral plasmonic nanostructures, the CD signals from chiral analytes are contaminated [5,16]. To solve this problem, achiral plasmonic nanostructures with strong optical chirality, such as double split-ring resonators (DSRR) [17], achiral plasmonic lattices [18], cavity-coupled achiral plasmonic system [19], and achiral nanorods [20–24], have been widely investigated. Among these achiral nanostructures, nanorods have a simple structure and can produce strong optical chirality under linearly polarized light, simplifying the fabrication and setup for CD measurements. The experiment works on enhancing chirality with nanorods using the near-field enhancement have also been reported [25]. Moreover, the handedness of the local optical chirality can be switched by the polarization of incidence, leaving no enantiomeric chiral nanostructure fabrication to achieve the opposite-handedness of the optical chirality. Therefore, achiral nanorods present a promising achiral platform for chiral sensing, which has not yet been experimentally demonstrated.
This study proposes an achiral nanorod with large near-field optical chirality under linearly polarized incidence. By changing the azimuth angles (the direction of incident polarization vs the long axis of the nanorods) from -90° to +90°, we theoretically achieved the tunability of the volume integral for an optical chirality from -4.4 to +4.4. Chiral molecular sensing was experimentally demonstrated by discriminating between the L- and D-phenylalanine (Phe) enantiomers. The CD value reached 1500 ± 100 mdeg in 0.2 g/L D-Phe solutions mixed with deionized water at a wavelength of 790 nm. Compare to CD signals of the chiral molecules in UV frequency, our device displays two orders of magnitude CD enhancement, which shows advantages over other achiral structures [25–32]. The minimal 0.05 g/L chiral molecular solution was experimentally detected. Our work demonstrates the promising potential of achiral nanorods for chiral sensing applications.
2. Results and discussion
The proposed device consisted of periodically arranged Au nanorods fabricated on a quartz substrate with a size of 100 µm × 100 µm, as shown in Fig. 1(a), where the light has normal incidence with linear polarization. Figure 1(b) displays the unit cell of the Au nanorod. The length L, width W, height H, and period P of the nanorods were 115 nm, 50 nm, 30 nm, and 300 nm, respectively. The azimuth angle θin is defined as the angle of the linearly polarized light relative to the long axis of the nanorod. By changing the azimuth angles, near-field optical chirality can be actively controlled using a highly local optical field. The Au nanorods were fabricated by electron beam lithography (EBL) followed by Au thermal evaporation and lift-off process (see details in the Methods section). The geometrical morphology of the periodic nanorods was measured by scanning electron microscopy (SEM), as shown in Fig. 1(c). Owing to the proximity effect, the nanorods present small distortions at the edges, which are inevitable in the nanofabrication process. The scale bar of the SEM image is 1 µm, and the inset is 100 nm.
Fig. 1. Proposed achiral nanostructure. (a) Schematic diagram of the achiral Au nanorods. (b) Illustration of the size and azimuth angle definition in the unit cell. (c) SEM image of the fabricated nanorods with a scale bar of 1 µm. The enlarged view shows a scale bar of 100 nm.
To explore the optical response of the nanorods, we simulated and measured the transmittance spectra at normal incidence with an azimuth angle of zero, as shown in Fig. 2(a). The transmittance dip at around 790 nm is due to the localized surface plasmon resonance (LSPR) of the nanorods excited along the long axis. Due to the small size of the nanorods, there are no higher-order modes on the nanorods. The resonant wavelength of the experiment was in good agreement with that of our simulation. Owing to the distortion and imperfection of the fabricated nanorods, a large scattering loss was induced in the experiment, leading to the experimental curve (red curve) presenting a wider full width at half maximum than the simulation. Figure 2(b) shows the azimuth-angle-dependent transmittance spectra, and as θin increased from 0° to 75°, the transmittance dip gradually disappeared, corresponding to the attenuation of the LSPR. Figure 2(c) shows the near-field electric field distributions at the resonance wavelength under a zero azimuth angle. The LSPR mode induces a 20× near-field enhancement with an electric field confined to the two ports of the nanorod. When the azimuth angle changes from 0° to 60°, the strength of the electric field gradually decreases, which is consistent with the far-field regularity, as shown in Fig. 2(c)–(e).
Fig. 2. Far- and near-field analysis of the achiral nanorods. (a) Simulated and measured transmittance spectra at normal incidence with an azimuth angle of 0°. (b) Simulated azimuth angle-dependent transmittance spectra. (c-e) Normalized electric field distributions at LSPR wavelength with azimuth angle ranging from 0° to 60°.
Furthermore, the near-field optical chirality of the achiral nanorods was analyzed. According to the definition of optical chirality, two prerequisites are required to achieve near-field optical chirality. First, the electric and magnetic fields must have parallel component, and these components must be π/2 out of phase. These prerequisites are satisfied by circularly polarized light rather than linearly polarized light. However, for our achiral nanorods, a large optical chirality could be obtained by linearly polarized light incidence, as shown in Fig. 3(a). To clarify the physical mechanism, an achiral nanorod at the LSPR wavelength can be expressed as a dipolar oscillation. According to the dipole model described in Ref. 24, the optical chirality can be described as [24]:
Fig. 3. Optical chirality of the achiral nanorods at different azimuth angles. (a) Simulated optical chirality with an azimuth angle of zero. (b-k) Simulated optical chirality around the nanorods with azimuth angle ranging from ±15° to ±75°. (l) Volume integral of the optical chirality over the nanorod as a function of the azimuth angles ranging from -90° to 90°.
Equation (2) shows that the underlying mechanism of the induced optical chirality in achiral nanorods is due to interactions between the y-component electric field of the dipole and y-component magnetic field of the incident light. As the Ed,y of the dipole shows different signs in each quadrant of the nanorods, the optical chirality shows a four-lobe distribution. The distributions of the near-field optical chirality normalized to the chirality of circularly polarized light (C0=±ω0|E0|2/2c) are also demonstrated by numerical simulation, which is highly consistent with the analysis of the dipolar model, as shown in Fig. 3(a). The optical chirality generated by the LSPR in achiral nanorods can exceed C0 by a factor of 25. Because of the symmetry of the configuration at the zero azimuth angle, the volume integral of C/C0 over the nanorod is zero, which cannot be used for chiral molecular sensing. By changing the azimuth angles, symmetry is broken, and a large-volume integral of the optical chirality is produced. Figure 3(b)–3(k) present the optical chirality at different azimuth angles ranging from ±15° to ±75°. For positive azimuth angles, the positive C/C0 (red area) at the corners becomes stronger than the negative C/C0 (blue area) at the other two corners, leading to a nonzero optical chirality positive volume integral ((C/C0)vol_int). For negative azimuth angles, a nonzero negative (C/C0)vol_int was obtained. As the azimuth angle increases, C/C0 decreases owing to the attenuation of the LSPR evanescent field. Therefore, the optimal azimuth angle is the trade-off between the symmetry and strength of C/C0. To qualitatively describe the azimuth angle dependence, the (C/C0)vol_int as a function of the azimuth angle ranging from -90° to 90° was calculated, as shown in Fig. 3(l). The maximal (C/C0)vol_int of up to ±4.4 is obtained at an azimuth angle of approximately ±45°, which is consistent with the experimental results of Ref. 21.
To demonstrate the chiral sensing performance, we employed our achiral nanorods to distinguish the chirality of the D- and L-Phe enantiomers. First, the chiroptical responses of D-, L-, and DL-Phe (racemic state) molecules in deionized water were measured using a CD spectrometer in the UV spectral range, as shown in Fig. 4(a). The molecules showed strong CD signals at wavelengths ranging from 180 to 225 nm, and the enantiomers presented complementary curves. In the racemic state, the measured CD signal was eliminated. At longer wavelengths, the chiroptical response of chiral molecules was too weak to be detected. Before the chiral sensing experiment using the achiral nanorods, we simulated the sensing performance of the achiral nanorods at azimuth angles of ±15°, ±25°, and ±35°, as shown in Fig. 4(b). Here, the chiral media was conducted by modifying the constitutive equations in COMSOL (see details in the Methods section). The transmission spectra under positive and negative azimuth angles were simulated, and CD was calculated using equation [7]:
where T± is the transmittance at positive and negative azimuth angles. For the D-Phe molecule, the CD can reach 1200 mdeg at a 35° azimuth angle (solid curves) if the azimuth angle is increased from 15° to 35°. The CD spectrum of the L-Phe molecule (dashed curves) shows complementary curves compared to that of the D-Phe molecule, as shown in Fig. 4(b). In Fig. 4(c), we experimentally performed chiral molecular sensing using our homemade free-space optical system. In our experiment, a circular rubber ring was used to hold a solution of chiral molecules (see details in the Methods section). A 0.2 g/L concentration of the D- and L-Phe molecules mixed with deionized water solutions was prepared for sensing. For D-Phe molecules, the CD values can reach 1500 ± 100 mdeg at an azimuth angle of 35°, as shown in Fig. 4(c). The shaded area displays the average measurement errors for five measurements. The measured errors are resulted from the both contributions of non-uniform of the molecules sticked to the nanorods [33] and our measured setup. For L-Phe molecules, the CD spectrum was complementary to that of D-Phe molecules, which is consistent with our simulated results. The slightly smaller CD values of L-Phe is attributed to the concentration deviation between D- and L-Phe molecules in the weighing process. In Fig. 4(d), we also measured the concentration dependence of CD at an azimuth angle of 25° at a wavelength of 790 nm, which shows good linear fitting for both D- and L-Phe molecules. Five concentrations were measured, including pure deionized water and 0.05 g/L, 0.1 g/L, 0.15 g/L, and 0.2 g/L solutions. The error bar was confirmed by five measurements, which showed small errors in the CD measurements. Therefore, we have demonstrated that the enhanced near-field optical chirality in our achiral nanorods can improve chiral molecular sensing performance at near-infrared wavelengths.Fig. 4. D- and L-Phe chiral molecule sensing using achiral nanorods. (a) CD spectra of D -, L -, and racemic DL-Phe molecules measured by CD spectrometer in the UV spectral range. (b) Simulated and (c) measured CD spectra of D- and L-Phe chiral molecules interacting with achiral nanorods at azimuth angles of 15°, 25°, and 35 °. The concentration of the chiral molecule solution was 0.2 g/L. (d) Measured CD as a function of the concentrations of the chiral molecule solutions from 0 to 0.2 g/L with an azimuth angle of 25°.
3. Conclusion
In conclusion, we experimentally demonstrated enhanced chiral sensing in achiral nanorods by linearly polarized light, eliminating the effects of structural chirality. Owing to the interactions between the electric field component of the LSPR mode and the magnetic field component of the incident light, the achiral nanorods exhibit large near-field optical chirality at the surface of the nanorods. Moreover, the signs of the volume integral of the optical chirality can be tuned by changing the azimuth angles, which provides a convenient method to detect the chirality of chiral molecules. Finally, D- and L-Phe chiral molecules were used to demonstrate the sensing performance of the achiral nanorods. The minimal 0.05 g/L solution of chiral molecules in deionized water was successfully detected by our achiral platform, with CD reaching 500 ± 11 mdeg. Therefore, our study provides an additional achiral nanophotonic platform for chiral drug sensing using an enhanced superchiral field.
Appendix A: optical and chiroptical simulation
The optical and chiroptical responses of the metamaterials were simulated using the commercial software COMSOL MULTIPHYSICS based on the finite element method. Periodic boundary conditions were applied along the X-and Y-directions to simulate a periodic structure. To avoid the reflectance of the port boundaries, perfect matching layers were used adjacent to the port. The permittivity of Au was fitted using the Drude model [34], and the silica substrate was set to a constant 1.46 [35]. The chiral molecules were dissolved in the deionized water, the refractive index of the molecular is nm = 1.33. The chiral medium was induced by changing the constitutive equations in the COMSOL software. The constitutive equations of the chiral medium are as [32]
where D is the displacement field, E is the electric field, H is the magnetic field, B is the magnetic flux density, ɛ0 and ɛr are the vacuum and relative permittivity, µr and µ0 are the relative and vacuum permeability, κ is the chiral parameter, and c is the speed of light. Owing to the weak chiroptical response in the near-infrared wavelength, κ is modeled as a constant with values of κ =±(5 + 0.005i)·10−5 over the simulated spectral range [7]. In the Wave Optics module of COMSOL, the detailed constitutive equations of chiral medium were modified as:The transmission spectra were calculated using the scattering parameters in COMSOL, and the CD was calculated using Eq. (3).
Appendix B: sample fabrications
We used electron beam lithography (EBL) to fabricate the samples. First, a poly(methyl methacrylate) (PMMA) resist was spin-coated on a silica substrate at a spin speed of 4000 rpm and baked for 3 min at 180 °C on a hot plate. Then, the structures were patterned by EBL with an accelerating voltage of 20 kV and an average dose of 160 µC/cm2. After exposure, the sample was developed in a solution of 3:1 isopropyl alcohol (IPA)/methyl isobutyl ketone for 1 min and rinsed in deionized water for 1 min. Subsequently, a layer of 5-nm Cr and 30-nm Au was deposited using a thermal evaporator (Leybold UNIVEX250). Finally, the PMMA resist was lifted off in acetone solution and rinsed in IPA.
Appendix C: characteristics of chiral sensing
The transmission spectra measurements were performed using homemade free-space optical equipment. The radius of the spot size was focused to 5 µm using a 10× objective lens (OptoSigma PAL-100-NIR). A calcite polarizer mounted on the rotation mount was used to produce linearly polarized light and change the polarized angle of incidence. For chiral sensing, a circular rubber ring adhered to the sample was used. Then, the prepared solution of the chiral molecule was dropped onto the ring covered with a silica substrate to hold the solution for the transmission spectra measurements. After each measurement, the sample was rinsed in deionized water for 5 min with ultrasonic cleaning. To confirm that the chiral molecules were removed, the transmission spectrum was measured and compared to the previous measurement to maintain it the same. Following these steps, solutions of the chiral molecules with different concentrations were measured.
Funding
Ministry of Science and Technology of the People’s Republic of China (MOST) (2018YFE0109200, 2021YFB2801600); National Natural Science Foundation of China (51972044, 52021001, 52102357); Sichuan Provincial Science and Technology Department (2019YFH0154, 2021YFSY0016, 2022YFG0076); Fundamental Research Funds for the Central Universities (ZYGX2020J005); Open Foundation of Key Laboratory of Laser Device Technology, China North Industries Group Corporation Limited (KLLDT202102).
Disclosures
The authors declare no conflicts of interest.
Data availability
All data are available from the authors upon reasonable request.
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