Distinct characteristics of DNA field effect transistors embedded with marine-derived porphyra-334 under UV illumination

Metabolites obtained from fungi, algae, and bacteria have attracted growing interests in the field of bionanotechnology due to their specific characteristics such as antioxidizing, antibiotic, and photo-protecting effects. Among them, mycosporine-like amino acids (MAAs) are water soluble and colorless metabolites found in marine and freshwater organisms. Currently, more than 20 different MAAs with molecular weights of < 400 Da are available. They have been emphasized owing to their specific physical capabilities. Substitution of various amino acids in the core structure of MAAs can produce a variety of MAAs having different absorption spectra owing to different functional groups attached to their rings. MAAs are known for their photo-protecting property as they can absorb potentially harmful wavelengths of UV radiation without degradation. In addition, they have photo-stability as they can dissipate energy without generating photochemical reactions. Consequently, MAAs can efficiently protect living organisms against UV radiation-induced stress.^1–7 Porphyra-334 derived from Porphyra yezoensis with characteristic absorption wavelength of 334 nm belongs to MAAs. It can absorb UV radiation due to the presence of a core central cyclohexanone or cyclohexenimine ring structure (conjugated with glycine/imino alcohols). It is supposedly to absorb UV radiation within the range of 309-360 nm and incorporate free radicals. Porphyra-334 has a hydroxyl ethyl group present in its core ring structure which has free π electrons and a lone pair of electrons.^8–11 

Although DNA is known to be the carrier of genetic information, it is also suitable for use in physical science and engineering. DNA (biodegradable and non-toxic polymer) can absorb UV light (∼260nm) due to the presence of π-stacked heterocyclic rings in the nucleotide chain. Functionalities and physical characteristics of DNA molecules can be enhanced by embedding various nanomaterials such as nanoparticles, dyes, carbon-based nanomaterials, ions, proteins, and MAAs into DNA molecules to aid binding and modification capabilities of DNA. In addition, methods to fabricate nanomaterial-embedded DNA complexes have been well developed. Therefore, researchers have started to use DNA as an efficient scaffold or template for various functionalized nanomaterials to construct useful physical devices and chemical/biological sensors.^12–18 

One of the most important and useful electronic devices might be a transistor because its logic implementation is easy and its information delivery capability can be controlled by current flow through gate. In addition, its fabrication technique is well-established. Field effect transistors (FETs) consisting of semiconducting silicon nanowires (SiNWs) with high detection sensitivity and silicon nanoribbons (SiNRs) having ambipolar characteristics have shown great potential in recognition of analytes such as ions, gases, proteins, and chemicals. Field effect governs the resulting current change when analytes (especially with biomolecules) interact with NWs/NRs, enabling us to detect minute charges at their interfaces to construct unique bioFETs.^19–22 

Although fabrication and characterization of metabolites, DNA, and FETs have been well established, construction of metabolite-embedded DNA FETs with SiNWs/SiNRs and study of physical characteristics of metabolite-embedded DNA FETs under ambient and specific conditions are rarely discussed due to the complexity of sample preparation and difficulties in measurements. By analyzing the conductivity of SiNW/SiNR FETs and electrochemical measurements of porphyra-embedded DNA (PDNA), distinct physical characteristics can be easily tuned which can lead us to construct useful devices and sensors. Here, we developed an efficient method to construct Porphyra-334 embedded DNA-coated SiNW FET (PDNA-SiNW FET) and SiNR FET (PDNA-SiNR FET). In addition, we measured and discussed current-voltage (I-V) characteristics of Porphyra-334 embedded DNA-coated SiNW FET (PDNA-SiNW FET) and SiNR FET (PDNA-SiNR FET) under UV light illumination and electrochemical cyclic voltammetry (CV) of DNA solution containing Porphyra-334.

DNA extracted from salmon was purchased from GEN Corporation (Shiga, Japan). DNA solution in deionized water at concentration of 5 mg/mL (i.e., 0.5 wt%) was prepared and stirred overnight to achieve a homogeneous mixture. Mycosporine amino acid Porphyra-334 extracted from Porphyra yezoensis was obtained from Korea Institute of Ocean Science and Technology (Ansan, Korea). A stock solution at 1 mg/mL was prepared in deionized water and stored at room temperature. Porphyra-334 embedded DNA solution was prepared by mixing the two solutions at 1:1 ratio.

SiNW FETs were fabricated on a silicon wafer with a back gate electrode and source-drain electrodes that were patterned using titanium (∼10 nm thick) and capped with a gold layer (∼25 nm thick). A total of 250 SiNWs were fabricated by thermally oxidizing the silicon surface, resulting in triangular-shaped nanowires with dimensions of 150 nm in width (≈ diameter), 10 μm in length, and spacing of 3.2 μm between them. SiNR-FET was also fabricated with 8-inch semiconducting process. A silicon substrate was used as a bottom gate electrode which had a silicon channel (∼20 nm thick) and a gate oxide layer of SiO₂ (∼5 nm thick).

Then 2 μL of DNA solution was dropped on either SiNW or SiNR FETs and left to dry at room temperature to construct DNA-coated SiNW FET (DNA-SiNW FET) or DNA-coated SiNR FET (DNA-SiNR FET), respectively. Similarly, 2 μL of Porphyra-334 embedded DNA solution was dropped on either SiNW or SiNR FETs for construction of PDNA-coated SiNW FET (PDNA-SiNW FET) or PDNA-coated SiNR FET (PDNA-SiNR FET), respectively (Figure 1(a, b)).

Electrical properties of DNA-SiNW/SiNR and PDNA-SiNW/SiNR FETs were determined using a semiconductor parameter analyzer (4200 SCS, Keithley Instruments Inc., OR, USA) under conditions of dark and UV light illumination (with wavelengths of 254 nm and 340 nm) (Figure 2 and Figure 4(a-d)).

Cyclic voltammogram (CV) and electrochemical impedance spectrum (EIS) under UV light illumination for DNA and Porphyra-334 embedded DNA solutions were obtained using a CS310 Electrochemical Workstation (Wuhan Corrtest Instruments Corp. Ltd., Wuhan, China) equipped with a three-electrode system. The system consisted of an Ag/AgCl reference electrode (RE), a platinum wire as a counter electrode (CE), and a gold PCB as the working electrode (WE). CV was recorded in a range of −0.6 V ∼ 0.6 V with various scan rates (S_r): 30, 50, and 100 mV/s. Impedance spectra were recorded within the frequency range of 0.001 Hz to 100 kHz with an AC amplitude of 10 mV and DC potential of 0.8 V applied to the WE held at an open circuit potential (Figure 1(c), Figure 3, and Figure 4(e, f)).

SiNW/SiNR FETs can detect the change of current through SiNW/SiNR channels induced by variation in charge density on the interface between SiNW/SiNR and deposited biomolecules under UV light illumination.²³ These SiNW/SiNR FETs (with a back gate electrode and source-drain electrodes) were fabricated on a silicon wafer (Figure 1(a, b)). Detailed geometry of FETs was discussed in materials and methods.^3,24 Figure 1(c) shows electrochemical measurement setup for obtaining CV and EIS under UV light illumination for DNA and PDNA solutions. The three-electrode system consisted of an Ag/AgCl reference electrode, a platinum wire counter electrode, and a gold PCB working electrode. The active area of the working electrode was 15 × 10 mm², which was large enough to detect cyclic current (I) and EIS responses for DNA and PDNA solutions with various scan rates (S_r) under UV light illumination as well as under dark condition.

Photocurrent measurements (I_DS) of SiNW FET and DNA-coated-SiNW FET at a fixed gate voltage (V_G = 5 V) are shown in Figure 2(a, b). I_DS was measured under UV wavelengths of 254 and 340 nm as well as under dark condition. Surface modification of SiNW with DNA molecules can result in modulation of charge carriers in SiNW channel.²⁵ Here, the channel can be assumed to be doped by DNA which affects the carrier mobility in a SiNW FET. Although there was a slight I_DS enhancement of DNA-SiNW FET than SiNW FET at UV having 340 nm, a difference of I_DS between SiNW FET and DNA-SiNW FET was hardly noticeable at 254 nm due to the characteristic absorption of UV light by DNA. DNA-SiNW FET under UV light illumination generated a large amount of charged carriers. Under UV light exposure, DNA-SiNW absorbs photons of energy higher than its band-gap, leading to an increase in I_DS. Based on measurement of I_DS-V_DS, we found that photoresistivities (ρ) were proportional to the area and inversely proportional to the length of SiNW. ρ values of SiNW FET (DNA-coated SiNW FET) were 6.08, 4.34 × 10^-1 and 3.89 × 10^-1 Ω.m (7.94, 4.15 × 10^-1 and 3.38 × 10^-1 Ω.m) at dark, 254, and 340 nm UV light illumination, respectively.

Figure 2(c, d) illustrates I_DS curves controlled by V_G measured at V_DS = 0.3 V for SiNR FET and DNA-SiNR FET under UV light illumination. SiNR FETs without and with DNA showed ambipolar behavior with unbalanced p- and n-branches. Interestingly, minute shifts in threshold gate voltage (i.e., Dirac point) of DNA-SiNR FET under illumination compared to SiNR FET were observed. This might be due to accumulation of photogenerated carriers between SiNRs and DNA caused by negatively charged DNA molecules. Graphene-based sensors and a hypothetical SiNR (demonstrated by density functional theory) designed to detect DNA molecules have shown similar behavior of threshold gate voltage shifts.²⁶ Minor deviation of Dirac points for SiNR FET was observed. It could be due to trapped charges in the oxide and the substrate.^27,28 Subsequently, photoconductivity (σ = 1/ρ) values calculated for SiNR FET (DNA-coated SiNR FET) were 6.18 × 10^-2, 5.91 × 10^-2, and 6.18 × 10^-2 S/m (6.39 × 10^-2, 6.33 × 10^-2 and 6.48 × 10^-2 S/m) at dark, 254, and 340 nm UV light illumination, respectively. The minute increase in conductivity of DNA-coated SiNR FET than SiNR FET was due to excess carriers provided by DNA molecules.

Electrochemical evaluation of DNA was carried out to observe the effect of UV illumination on redox potentials of DNA. Oxidation of DNA occurs majorly via hole transfer along the π-stack interaction followed by yielding a radical cation from bases of DNA.^29,30 Cyclic I response and EIS in the Nyquist plot at dark and UV illuminating conditions are shown in Figure 3. DNA solution was filled in a transparent container equipped with working, counter, and reference electrodes. Impedance was measured in the frequency range of 1 mHz ∼ 100 kHz while CV was measured at varied scan rates (S_r) of 30, 50 and 100 mV/s under illumination of UV light with wavelengths of 254 and 340 nm.

At dark condition, cyclic I revealed oxidation occurring in DNA (revealed as the peak arising at ∼0.45 V) and reduction occurring at the working electrode (shown as the peak at around −0.36 V) (Figure 3(a)). As expected, cyclic I amplitude was proportional to S_r. Figure 3(b) displays cyclic I response for DNA solution under UV light illumination at a fixed S_r. A scan rate of 50 mV/s was chosen because well-defined peaks were obtained during measurement. Under UV illumination, an oxidation peak was shifted from 0.45 V (at dark) to 0.30 V. The shift in peak location might be attributed to the enhanced oxidation of DNA which occurs while absorbing UV light. It meant that irreversible DNA oxidation was enhanced under UV light illumination. Electrochemical impedance measurement of DNA was carried out in deionized water which had a relatively higher resistive nature (18 MΩm). Nyquist plot (i.e., real vs imaginary impedance plot) in Figure 3(c) shows an equivalent series resistance (R_S, corresponding to a starting point of real impedance) of 2.74, 2.50, and 2.86 kΩ with charge transfer resistance (R_P, indicating the radius of real impedance) of 29.86, 29.07, and 29.14 kΩ under dark, 254, and 340 nm UV illumination, respectively. The inset shows an equivalent circuit for the system. DNA being an electrolyte of low ionic strength resulted in large impedance. A slope in Nyquist plot indicates the degree of charged molecular diffusion. Consequently, higher slopes of DNA obtained under UV illumination indicated improved diffusion process during measurement.

I_DS-V_DS characteristics and ρ of PDNA-SiNW FET at a fixed gate voltage V_G = 5 V under UV light illumination were obtained (Figure 4(a, b)). Enhancement of I upon illumination with UV light was evident in PDNA-SiNW FET because of injection of excess charge carriers from PDNA on SiNW FET by absorbing UV light. A bar graph in Figure 4(b) represents comparative ρ of SiNW, DNA-SiNW, and PDNA-SiNW FET at dark and UV light conditions measured at V_DS = 1.0 V. Under UV illumination, ρ of all samples was reduced significantly. Noticeable reduction of ρ was observed by addition of Porphyra-334. The ρ reduction of 67% (from 6.08 Ωm through bare SiNW FET to 1.98 Ωm via PDNA-SiNW FET), 34% (from 4.34 × 10⁻¹ to 2.87 × 10⁻¹ Ωm), and 39% (from 3.89 ×10⁻¹ to 2.37 × 10⁻¹ Ωm) at dark, 254, and 340 nm UV illumination were obtained, respectively. Nonradiative resonance energy transfer (NRET) between DNA and Porphyra-334 might play a role in the increase of I (consequently the decrease in ρ).³¹ 

In addition, I_DS and conductivity (σ) at V_DS = 0.3 V of a PDNA-SiNR FET under UV light illumination were obtained (Figure 4(c, d)). Due to embedment of Porphyra-334 in DNA (p-doping), a large threshold voltage shift towards the negative scale was observed under all conditions. Consequently, the energy level of PDNA was modified to a lesser value which increased the electric field in the SiNR followed by enhanced charged carrier mobility. Comparison to the dark condition, a positive threshold voltage shift was observed upon UV light illumination which could be due to enhanced electron flow in PDNA upon absorption of UV light. σ of SiNR (obtained from I_DS-V_G in Figure 2(c)), DNA-SiNR (Figure 2(d)), and PDNA-SiNR (Figure 4(c)) FETs measured at V_G = 30.0 V are displayed in Figure 4(d). Increment in σ of PDNA-SiNR FET under UV illumination (especially at wavelength of 340 nm) compared to bare SiNR FET was noticed due to excess charge carriers provided by PDNA via absorbing UV light (similar to the characteristic absorption wavelength of Porphyra-334). Increases in σ from SiNR FET to PDNA-SiNW FET at dark (from 6.18 × 10⁻² to 7.47 × 10⁻² S/m), 254 nm UV (5.91 × 10⁻² to 1.01 × 10⁻¹ S/m), and 340 nm UV (6.18 × 10⁻² to 1.34 × 10⁻¹ S/m) were noticed.

Figure 4(e, f) shows CV (obtained at a fixed S_r = 50 mV/s) and impedance spectra of PDNA solution in Nyquist plot under UV illumination. The mixing of Porphyra-334 with DNA gave rise to an unreactive species as a result of p-doped DNA by Porphyra-334. Due to absorption of UV radiation without degradation and dissipation of energy without generating photochemical reactions, oxidation of DNA at 0.3 V was suppressed after the addition of Porphyra-334 to DNA solution, indicating a photoprotective effect of Porphyra-334 on DNA. A Nyquist plot and bar graph showing the change in R_S for PDNA solution are shown in Figure 4(f). Here, DNA served as an electrolyte with low ionic strength. Due to increase of the ionic strength by addition of Porphyra-334, PDNA solution showed significantly low R_S compared to DNA. A bar graph in inset displayed a decrease in R_S values for PDNA compared to DNA from 2.74 to 0.31 kΩ at dark, 2.50 to 0.30 kΩ at 254 nm UV, and 2.86 to 0.29 kΩ at 340 nm UV. Finally, we obtained R_P by measuring radii of real impedances in the Nyquist plot which corresponded to 14.87, 14.86, and 14.87 kΩ for PDNA under dark, 254, and 340 nm UV illumination, respectively.

In conclusion, an effective method to construct PDNA-SiNW/SiNR FETs was developed. Current-voltage characteristics of PDNA-SiNW/SiNR FETs and electrochemical cyclic voltammetry of DNA solution containing Porphyra-334 under UV light illumination were investigated. Porphyra-334 embedded DNA-coated SiNW/SiNR FETs are able to detect change of current via the interface between SiNW/SiNR and the deposited biomolecules. We studied I-V characteristics and conductivities of PDNA-SiNW and PDNA-SiNR FETs under UV light illumination. Conductivities of PDNA-SiNW and PDNA-SiNR FETs under UV illumination were enhanced by the addition of Porphyra-334. The cyclic voltammogram and impedance spectra of DNA solution with Porphyra-334 under UV illumination were also investigated. The addition of Porphyra-334 to DNA solution suppressed its irreversible oxidation, indicating the photoprotective effect of Porphyra-334 on DNA. Furthermore, the ionic strength of DNA solution was increased with further addition of Porphyra-334, leading to lower resistance. Our approach might lead to a new path to construct highly efficient FETs capable of detecting biomolecules in the future.

This research was a part of the project titled Development and Industrialization of Marine derived MAA for Anti-aging Cosmetic Products (20150071), funded by the Ministry of Oceans and Fisheries, Korea. In addition, it was supported by grant (2018R1A2B6008094) of the National Research Foundation (NRF) of Korea.