Biopolymer materials show promise for electronics and photonics applications

Nano/biotronics is a new research area that uses biologically based materials and devices for photonics and electronics applications. This is very different from biophotonics, in which photonics technologies are used for medical applications. Nano/biotronics has shown that biotechnology is not only applicable to genomic sequencing and clinical diagnosis and treatment. It can also have a major impact on nonbiotech applications, opening up a whole new field for bioengineering.

Working with the Chitose Institute of Science and Technology, the Asian Office of Aerospace Research and Development, and the Air Force Office of Scientific Research, the US Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/RX), developed a new class of polymer based on deoxyribonucleic acid (DNA) derived from salmon milt and roe sacs, biowaste material left over from processing the edible parts of the fish.^1,2 The DNA is purified via an enzymatic process in which 98% of the proteins are removed, decolorized with a carbon treatment, and freeze-dried (see Figure 1).

Figure 1. The process of processing and purifying salmon DNA.

This new DNA-based biopolymer material possesses unique optical and electromagnetic properties, including low and tunable electrical resistivity, and ultralow optical and microwave loss.^3–47 Organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and nonlinear optical (NLO) polymer electro-optic (EO) modulators fabricated from this new biopolymer have demonstrated performance that exceeds that of state-of-the-art devices made with currently available organic-based materials.^{24–27,33,37}

In addition to its unique electromagnetic and optical properties, the DNA-based biopolymer is abundant, inexpensive, replenishable, and composed of green materials. A variety of agricultural waste products can be used as raw materials, and because the biopolymer is not fossil fuel-based, it will not deplete natural resources or harm the environment. Currently available polymer materials either have low optical loss and high electrical resistivity or low electrical resistivity and high optical loss. Our DNA-based biopolymer, on the other hand, has shown both low optical loss and low electrical resistivity.

Optical losses ranging from 0.1 to 1.2dB/cm have been observed over a broad wavelength range, 600–1700nm. At the same time, by reducing the molecular weight, we have increased the conductivity of the material by seven orders of magnitude without increasing the optical losses. An electrical resistivity of 10⁷Ω-cm has been measured for the reduced molecular weight biopolymer. Therefore, the resistivity can be tuned via control of the molecular weight.^{17,24–28,33,37}

This material has been found to have high thermal stability up to a temperature of 230°C, and it maintains its double-helical structure to temperatures in excess of 100°C.^10,27,33,37 The electrical resistivity measures three to five orders of magnitude lower than that of other polymer materials, and its optical and microwave losses are an order of magnitude lower.^5,13 These characteristics make the biopolymer very attractive for electro-optic devices.

Work in conjunction with the University of Southern California, the University of Washington, the University of Cincinnati, and the University of Dayton first employed this new material in cladding layers for NLO polymer EO modulators (see Figure 2).^8–12 Through collaboration with IPITEK Corp., optical insertion losses were reduced from 13–15 to 10dB.²⁶ This is an improvement of two to three times. Taking advantage of the biopolymer's tunable resistivity and low optical loss, AFRL/RX and the University of Dayton created the first all-DNA EO modulator. We used DNA biopolymer doped with disperse red 1 (DR1) for the core layer, realizing similar nonlinearity compared with other polymer hosts, but with reduced optical losses (see Figure 3).^27,33,37

Figure 2. Nonlinear optic (NLO) polymer electro-optic (EO) modulator with an LD3-based NLO polymer core layer and DNA-based top and bottom cladding layers.

Figure 3. The all-DNA EO modulator exhibits smaller optical losses compared to other polymer hosts.

Using a DNA-based biopolymer for the dielectric layer in an organic field effect transistor (OFET), the University of Linz, AFRL/RX, and the European Office of Aerospace Research and Development demonstrated a BioFET that operated at a gate voltage nearly an order of magnitude lower than OFETs using other dielectric polymers, such as polyvinyl alcohol, for the gate dielectric (see Figure 4).²⁵ We predicted a factor of two improvement, so we seem to have benefited from using DNA in another, not yet understood, way.

Figure 4. A field effect transistor using a DNA-based biopolymer gate dielectric (BioFET) operates at a lower gate voltage than organic field effect transistors using other dielectric polymers.

Figure 5. Green organic light-emitting diodes (OLEDs): (a) without an electron blocking layer (EBL); (b) with a DNA-based EBL (BioLED).

AFRL/RX and the University of Cincinnati used the DNA-based biopolymer as an electron-blocking layer (EBL) in fluorescent organic light-emitting diodes (OLEDs). We demonstrated red, blue, and green BioLEDs that were as much as 30 times brighter and operated at 10 times the efficiency compared to OLEDs without the biopolymer EBL.^24,29,31 Figure 5 presents photographs of green OLEDs with and without the DNA-based EBL. More recently, AFRL/RX and the University of Cincinnati showed that DNA-based biopolymers can be vapor-deposited, achieving a world record 15cd/A efficiency for fluorescent OLEDs.³⁹ This efficiency is twice as high as that of the BioLED with spin-deposited DNA biopolymer.

Figure 6. Measured (open circles) and fitted (solid curve) third harmonic generation (THG) intensities as a function of the incidence angle for (a) a glass substrate, (b) a glass substrate with a DNA-based thin film, and (c) a glass substrate with a 5% DR1-DNA-based thin film.

Working with the Université d'Angers and the European Office of Aerospace Research and Development, we demonstrated significantly increased third-order nonlinearities when the DNA-based biopolymer was used as the host material.⁴⁴ In the case of DNA-based films, we observe the value of third harmonic generation (THG) susceptibility to be about one order of magnitude larger than that of silica. This difference may be due to the presence of highly polarizable conjugated π electrons in DNA. For the DNA complex doped with only 5% of DR1, we observed a THG susceptibility two orders of magnitude larger than fused silica (see Figure 6). This is the first reported observation of purely electronic, fast NLO susceptibility in a DNA-based complex thin film.

Figure 7. Experimental setup for two-photon lasing from a DNA-chromophore complex.

Figure 8. Electric field-induced dielectric tuning of the DNA biopolymer is comparable to barium strontium titanate.

The University at Buffalo and AFRL/RX showed enhanced infrared two-photon excited visible lasing from a DNA biopolymer-chromophore complex.²² The DNA-based biopolymers could be doped at a much higher level without aggregation than other polymer host materials, such as polymethylmethacrylate (PMMA) (see Figure 7). AFRL/RX, with the University of Dayton, showed that an applied electric field can tune the dielectric constant of the DNA biopolymer, changing it as much as 52% with an applied field of 113kV/cm (see Figure 8).^38,42,47 This is comparable to barium strontium titanate. Using a technique known as electrospinning, the University of Connecticut and AFRL/RX demonstrated a 20-fold enhancement in the photoluminescence of fluorescent dye-doped DNA biopolymer nanofibers as compared to spin-deposited fluorescent dye-doped DNA biopolymer films.⁴⁶

Working with the University of Cincinnati, we also found enhanced photoluminescence and lasing from a sulforhodamine (SRh)-doped DNA biopolymer.^35,43 The maximum emission was obtained with 1wt% SRh in DNA, equivalent to 100 DNA base pairs per SRh molecule. Using a distributed feedback grating structure with a period of 437nm, which corresponds to a second-order emission at the amplified spontaneous emission wavelength of 650nm, a lasing threshold of 3μJ was achieved, corresponding to ∼30μJ/cm² or 4kW/cm². The slope efficiency of the lasing was ∼1.2%. The photoluminescent intensity of the DNA-SRh was 17 times higher than that of PMMA-SRh (see Figure 9).

Current efforts include tailoring DNA's electromagnetic properties for electronics applications using metal nanoparticle- and carbon nanotube-doped DNA with Korea University, the University of Cincinnati, the University of Dayton, and the Air Force Office of Scientific Research; exploring chirality's role in negative refractive index with the Australian National University and the Asian Office of Aerospace Research and Development; and investigating the photo, chemical, and thermal degradation of doped and undoped DNA-based biopolymers with the Politehnica University of Bucharest and the European Office of Aerospace Research and Development.

Figure 9. The lasing from a DNA-based sulforhodamine-doped SNA biopolymer was much higher than that from a polymethylmethacrylate (PMMA)-based sulforhodamine material.

In August 2007 SPIE kicked off a new conference on Nano/biotronics at Optics+Photonics in San Diego. Emily Heckman (General Dynamics Information Technology), Junichi Yoshida (Chitose Institute of Science and Technology), and Birendra Singh (University of Linz) served as chairs for this first conference of its kind. The second conference on Nano/biotronics is scheduled for August 2008, again in San Diego.

In summary, nano/biotronics, or nano/biopolymer engineering technology, has the potential to revolutionize next-generation polymers and organic-based devices. DNA-based biopolymers show great promise for numerous optical, electronic, and electro-optic applications, with demonstrated increases in material properties and device performance. Indeed, these materials could prove tomorrow's ‘silicon’ of polymers.