3.4.2. Laser Synthesis of Graphene

Laser printing is an emerging technology whereby a thin film of material is selectively removed from a carrier substrate via a laser beam and irradiated to a receiver substrate [123]. This approach allows for integration with roll-to-roll laser printers, printing without harsh chemicals, high spatial resolution, and control of edge plane functionalization, which makes laser printing of great interest for bioelectronics applications [123,124]. For instance, Rahimi et al. demonstrated a high throughput process by which graphene can be irradiated onto a PDMS substrate to yield strain sensors sensitive up to 100% strain with a gauge factor of up to 20,000 [124]. It was reported that laser power and speed greatly affected print quality and conductivity, and the authors optimized the process to 0.5–1.9 m/s and 4.5–8.25 W for printable traces [124]. Laser printed graphene is also of great utility in a number of biosensor applications. For instance, Ortiz-Gómez et al. ablated a PI film with a 12 W CO<sup>2</sup> laser operating at 2.4 W and 0.15 m/s to create a graphene heater for a microfluidic device that used fluorescent silicon nanodots to detect total carbohydrates [125]. In addition, GO can be reduced by laser excitation through the conversion of sp<sup>3</sup> carbon to sp<sup>2</sup> and the removal of oxygen functional groups, and the photothermal and photochemical processes involved in the reduction of GO can be well controlled by altering the laser wavelength [126]. For instance, Zahed et al. used a CO<sup>2</sup> laser to reduce GO for an electrocardiography (ECG) sensor with comparable signal quality to commercial Ag/AgCl electrodes (12.9 dB vs. 13.3 dB) [127].

#### 3.4.3. Post Print Processing

Because graphene ink stability is predicated heavily on the addition of strong solvents and polymer stabilizers, these chemicals must be evaporated, dissolved, decomposed, or otherwise removed in order to yield optimal conductivity and material properties [128]. Post-print processing is highly dependent on the choice of chemical additives. For instance, graphene dispersed in high concentrations of EC must be treated at 300–400 C, whereas EG-PVP mixtures can be cured at 120 ◦C [82]. Furthermore, several inks that do not evaporate solvents can be treated at room temperature, although these inks will exhibit lower conductivities as a result [129,130]. While thermal curing beyond 120 ◦C is not suited for flexible electronics on many substrates, such as PET and TPU, novel laser treatment approaches are able to efficiently treat printed patterns without damaging the underlying substrate [131]. For instance, Jabari et al. reported a laser treatment method to cure printed graphene with similar conductivities to traditional thermal curing [131]. Likewise, Secor et al. demonstrated an intense pulsed light annealing for inkjet-printed graphene that is suited to a variety of substrates and can result in fewer impurities than thermal alternatives [132]. Finally, GO is much easier to disperse than graphene, but it must be reduced after printing with harsh chemicals and high temperatures, which often results in defects and poor conductivity [128]. As a result, GO is not as attractive as graphene for bioelectronics applications.

**Figure 7.** Metal nanowire synthesis, printing, and welding. (**a**) SEM images of Ag nanowires synthesized at different PVP:AgNO3 molar ratios of (**i**) 3:1, (**ii**) 4.5:1, (**iii**) 6:1, (**iv**) 7.5:1, (**v**) 9:1, and (**vi**) 11:1. All scales are the same. Changes in (**vii**) nanowire diameter and (**viii**) length with PVP:AgNO3 molar ratio are also shown. (reprinted with permission from *Cryst. Growth Des*. (**2011**), 11, 11, 4963–4969, Copyright 2011, ACS). (**b**) Images of gravure printed AgNW traces with various thicknesses and an SEM image demonstrating the aligned AgNW network. (reprinted with permission from *Sci. Rep*. (2018), 8, 15167, Copyright 2018, Nature Publishing Group). (**c**) SEM image of CuNWs nanowelded with laser irradiation (**i**) and TEM images of CuNWs before (**ii**) and after (**iii**) laser irradiation. (reprinted under Creative Commons license CC BY from *Sci. Rep.* (2017), 7(1)). (**d**) SEM images of sparse, randomly oriented AgNWs on a PET film (reprinted courtesy of *Org. Electron*. (2014) 15(11), 2685–2695, Copyright 2014, Elsevier).

## 3.4.4. Graphene Functionalization for Biosensor Applications

Organo-functionalized graphene has played a crucial role in the development of novel biosensors, and the ability to print such sensors in roll-to-roll methods would have a transformative effect on healthcare [12]. Although each of the four materials covered in this review has been successfully explored biosensors, those based on graphene have generated the most recent interest because of the great degree of freedom in material functionalization [15]. Graphene functionalization occurs either covalently or non-covalently.

In covalent functionalization, graphene is oxidized to GO, and covalent bonds are formed to organic functional groups on a sensing material [128]. For instance, a carboxylic group on GO can covalently bond to glucose oxidase to form a glucose sensor [12,16,128]. Examples of covalent sensing systems on functionalized GO are illustrated in Figure 8c [13]. Non-covalent bonding occurs when functional groups are attracted to graphene through Van der Waals and electrostatic forces, but this bonding is typically nonstable for long durations [12,133]. Instead, target biomolecules may be directly absorbed into the graphene, allowing the graphene to serve as a sensor through non-covalent functionalization [133]. The most common form of glucose-based biosensors is electrochemical sensors. Graphene functionalized with biological receptors is employed as a working electrode to detect analytes through electrochemical oxidation or reduction of analytes [12]. For instance, Kinnamon et al. screen-printed GO on a textile substrate and bound 1-Pyrenebutyric acid-*N*-hydrosuccinimide ester (PANHS) as a crosslinker to bind to an influenza A-specific antibody [16]. The textile sensor demonstrated high stability with washing (~4.6% variability) and accurate sensing over a range of virus expression of 10 ng/mL to 10 µg/mL with a limit of detection of 10 ng/mL. The sensor also exhibited very good specificity, and the sensing range is well suited to the average human viral expression of 50 ng/mL [16]. In addition, graphene field effect transistor (FET) biosensors may be used to control the flow of current as a function of charge accumulating on a functionalized graphene gate of channel, as shown in Figure 8d [12]. For instance, Xiang et al. used inkjet printing to deposit a graphene channel for a fully printed FET on the PI with low resistivity (110 Ω/sq) that was subsequently functionalized in cystamine solution (Figure 8e) [133]. Norovirus antibodies were then bonded, and bovine serum albumin was introduced to prevent nonspecific binding of other biomolecules. It was determined that the voltage gain from source to drain with an applied 10 GHz wave generates a linear response from 0.07 to 3.70 dB when the concentration of Norovirus protein increases from 0.1 to 100 µg/mL [133].
