Two-Dimensional Biocompatible Plasmonic Lenses for Color Blindness Correction


 Color blindness, or color vision deficiency (CVD), is an ocular disease that suppresses the recognition of different colors. Recently, tinted glasses and lenses have been studied as hopeful devices for color blindness correction. In this study, 2D biocompatible and flexible plasmonic lenses were fabricated using polydimethylsiloxane (PDMS) and an innovative, low-cost, and simple design based on the soft nano-lithography method. These lenses were investigated for correction of red-green (deuteranomaly) color blindness. The plasmonic lens proposed herein is based on the plasmonic surface lattice resonance (SLR) phenomenon and offers a good color filter for color blindness correction. The biocompatibility, low cost, and simple fabrication of these contact lenses can offer new insights for applications of color blindness correction.


Introduction
Human color vision originates from three types of cone-shaped photoreceptors, i.e. short (S), medium (M), and long cones (L) which are responsible for perceiving blue, green, and red colors with spectral sensitivity peaks around the 430, 530, and 560 nm, respectively [1].
Color blindness, or color vision de ciency (CVD), is an ocular disease that prevents the recognition and perception of speci c colors by three photoreceptors which, in normal vision, are all present and function according to their spectral sensitivity peaks. This ocular disorder can be either acquired or congenital and is caused by the lack of or a de ciency in the cone-shaped photoreceptors [2].
There are three different types of color blindness: dichromacy, monochromacy, and anomalous trichromacy [3]. Dichromacy occurs when one of the cone-shaped photoreceptors is completely missing and is categorized as protanopia (missing red cone cells), deuteranopia (missing green cone cells), or tritanopia (missing blue cone cells). Monochromacy is the rarest type of color blindness in which at least two cone-shaped photoreceptors are missing. Monochromat people are completely colorblind (achromatopsia) or have only blue cone-shaped photoreceptors. As the third classi cation, anomalous trichromacy occurs when one of the cone-shaped photoreceptors is defective. Depending on which coneshaped photoreceptor is defective, anomalous trichromacy is divided into three categories: protanomaly (defective red cone cells), deuteranomaly (defective green cone cells), and tritanomaly (defective blue cone cells).
The most common types of color blindness are protans (protanopia and protanomaly) and deutans (deuteranopia and deuteranomaly), which are known as red-green color blindness [4]. The spectral sensitivity peak of the red cones is blue-shifted in protanomaly, while the sensitivity peak of the green cones is red-shifted in deuteranomaly. Thus, patients cannot distinguish different colors due to overlapping in the spectral sensitivity of green and red cones.
Tinted glasses with color lters for color blindness correction have been widely investigated and are even commercially available [10][11][12][13]22]. While these glasses are effective for improving the color perception of color-blind people, they also have limitations such as high cost, bulkiness, and incompatibility with other vision correction glasses.
Recently, contact lenses based on chemical dyes [23][24], plasmonic metasurfaces [25], and plasmonic nanoparticles [26 -27] have been investigated for color blindness correction. However, these contact lenses face challenges such as non-biocompatibility, short time usability, low stability, high cost, and the complexity of the fabrication process.
In the current study, 2D biocompatible and exible plasmonic lenses based on polydimethylsiloxane (PDMS) are proposed for color blindness correction, and speci c consideration is given deuteranomaly (red-green) color blindness, which is the most common type of color blindness. PDMS is a biocompatible, exible, and transparent material which can be a good candidate for fabricating contact lenses. This nontoxic and biocompatible material has attracted many applications in elds such as biology [28-32], medicine [33-34], and chemistry [35]. In this work, 2D exible and biocompatible PDMS-based lenses were successfully fabricated using an innovative, low-cost, and simple design based on the soft nanolithography method and investigated for correction of red-green color blindness. The biocompatibility, low cost, and simple fabrication of these contact lens can offer new insights into applications for color blindness correction.

Experimental Method
The PDMS-based lenses and also two dimensional biocompatible plasmonic lenses proposed herein were fabricated as two separate lenses with poly-dimethylsolaxane (PDMS) (SYLGARD 184 DOW CORNING). First, the proposed lenses were fabricated according to the schematic diagram shown in Fig.   1(a). First, PDMS was prepared by combining it with a curing agent at a weight ratio of 10:1. After mixing, these two parts by DC mixer for 5 minutes to achieve a homogenous mixture, which was poured onto the lens mold. For degassing, the mold was placed in a vacuum chamber for 15 min. Afterward, the sample was placed on a heater and cured with gradual increases in temperature from 50 0 C to 100 0 C over 1 h.
After 24 h, the PDMS-based lens was peeled off from the mold, and thus, a biocompatible PDMS-based lens was successfully produced ( Fig. 1(b)). In the next step, the fabricated PDMS-based lenses were immersed into 0.01 M gold solution (HAuCl 4 ·3H 2 O gold chloride trihydrate) at different incubation times of 12, 18, 24, and 36 hours ( Fig. 1(c)) and then investigated for color blindness correction.
As the second proposed structure, the 2D biocompatible plasmonic lenses were fabricated using the soft nano-lithography method and investigated for correction of red-green color blindness. Contact lenses should be curved due to the natural curvature of the cornea, and conventional lithography methods are only applicable for at and planar substrates. In this research, the innovative, simple, and low-cost technique based on soft nano-lithography method was suggested to create a two-dimensional plasmonic nanostructure onto the curved surface of the lens. In this method, the charge-coupled device (CCD) of a camera was extracted and utilized as a stamp. The CCD camera had a two-dimensional periodic square pattern with a periodicity of 2.5μm. The CCD stamp was placed into the central part of the lens mold, and a mixture of PDMS and curing agent was poured onto it ( Fig. 2(a)). After degassing as mentioned in the previous step, the lens mold was placed on a heater and cured with gradual increases in temperature from 50 ℃ to 100 ℃ over a period of 1 h. Finally, the PDMS-based lens was separated from the mold after 24 h, and the 2D PDMS-based lens was gently peeled off of the CCD stamp. A gold layer with a thickness of 35 nm was deposited onto the patterned central part of the PDMS-based lens using the PVD technique.
In this way, a 2D exible and biocompatible lens was successfully fabricated with an innovative, low-cost, and simple design method compared to the other costly and complex methods, such as the electron beam lithography technique. An image of the actual fabricated 2D biocompatible plasmonic lenses is shown in Fig. 2(b).
To characterize these lenses, we use Scanning Electron microscopy (SEM) and UV-Vis absorption spectroscopy.

Results And Discussion
An image of the actual fabricated plasmonic PDMS-based lenses with different immersion times into HAuCl 4 .3H 2 O gold solution is shown in Fig. 3(a). As shown, the color of the lenses changed with increases in immersion time, which corresponds to increases in the Au NPs content of the PDMS-based lenses. The absorption spectra of the proposed lenses with different immersion times were measured using a UV-Vis spectrometer and are shown in Fig. 3(b). As can be seen, the value of the absorption peak was enhanced with increases in immersion time, which corresponds to increases in the percentage of Au NPs in the PDMS-based lenses. In addition, absorption peaks due to plasmonic resonances were observed at λ = 532, 533, 535, 542, 543 nm for lenses with immersion times of 12, 18, 24, 36, and 72 h, respectively. Therefore, the absorption resonance peak has a red shift of about 11 nm with increasing the immersion time from 12 h to 72 h. Also, the plasmonic resonance peak only has a red shift of 1 nm with increasing incubation time from 36 to 72 h, and no signi cant change in the wavelength location of the resonance response was observed.
This effect indicates the stabilization of the AuNPs trapped inside the PDMS-based lens after an incubation time of 36 h; therefore, the optimum immersion time is 36 hours.
The proposed plasmonic lens is based on the tunable localized surface plasmon resonance (LSPR) phenomenon. Plasmonic Au NPs embedded in the fabricated PDMS-based lens offer a good color lter for color blindness correction. In addition, the optical LSPR properties of gold NPs can be adjusted by controlling their morphology, including size, shape, and solvent.
The wavelength range of 540-580 nm (problematic wavelength range) must be ltered to correct deuteranomaly (red-green) color blindness, so the resonance peak must occur at the wavelength of about 560 nm. For this purpose, the concentration of the HAuCl 4 .3H 2 O gold solution was increased from 10 to 25 mM, and a PDMS-based lens was immersed in the 25 mM gold solution (HAuCl 4 ·3H 2 O gold chloride trihydrate) for 36 h. For better comparison, the absorption spectra of the lenses that were immersed in the 10-and 25-mM gold solutions are given in Fig. 3(c)). As seen, the plasmon resonance peak has a red shift of about 11 nm with increasing the concentration of the HAuCl 4 .3H 2 O gold solution from 10 to 25 mM, and plasmonic resonance occurred at λ = 553nm. In fact, the size of the Au NPs increased with increases in concentration, so the resonance wavelength had a red shift. Furthermore, the value of the absorption peak was enhanced with increases in the concentration of the HAuCl 4 .3H 2 O gold solution.
The scanning electron microscopy (SEM) image of the fabricated 2D plasmonic lens is given in Fig. 4(a). As can be seen, the proposed 2D plasmonic lens has a two-dimensional periodic square pattern with high resolution. Additionally, the absorption spectrum of the fabricated 2D plasmonic lens was measured using a UV-Vis spectrometer and is shown in Fig. 4(b).
Metallic nanoparticles arranged in a periodic array can exhibit extremely narrow and strong excitations known as plasmonic surface lattice resonance (SLR) [36][37]. This phenomenon is a result of the coupling between the diffracted order (DO) waves in a periodic structure and the localized surface plasmon resonances (LSPRs) coming from nanowires at the corners of each unit cell. The proposed 2D plasmonic lens is composed of a two-dimensional array of Au NWs and can support the sharp diffracted order (DO) waves and LSPR modes. As seen in Fig. 4(b), the absorption peak occurred at λ = 560nm, which corresponds to the plasmonic surface lattice resonances (SLR) caused by plasmonic 2D array of the lens, and this proposed lens offers a good color lter for the correction of deuteranomaly color blindness.

Conclusion
In sum, 2D biocompatible and exible plasmonic lenses based on polydimethylsiloxane (PDMS) were fabricated with an innovative, low-cost, and simple design based on the soft nano-lithography method and investigated for correction of red-green color blindness. PDMS, a biocompatible, nontoxic, exible, and transparent material, was used to fabricate the lenses. This proposed plasmonic lens is based on the plasmonic surface lattice resonance (SLR) effect and can be utilized as a good color lter for the correction of deuteranomaly color blindness. Furthermore, the proposed lens offers excellent properties such as biocompatibility, stability, and exibility, which can be useful for applications of color blindness correction.