phthalocyanine and a perylene tetracarboxylic derivative [42]. This, and a plethora of

other works, have certainly inspired the use of organic dyes and pigments in bioelec­

tronics. A recent example is a study reported by Rand et al., in which the authors used a

p-n junction consisting of nontoxic and commercially available pigments (phthalocyanine

and N,N′-dimethyl perylene-3,4:9,10-tetracar-boxylic diimide) to photostimulate neurons

through photocapacitive effects [43]. By exploiting the same stimulation approach, this

research group has also built up organic electrolytic photocapacitors to generate capa­

citive current for X. laevis oocyte stimulation [44].

Apart from synthetic systems, nowadays there is a growing interest in naturally oc­

curring organic electronic materials. For instance, melanin derivatives represent a pop­

ular class of naturally occurring organic pigments for bioelectronic and optoelectronic

devices, owing to their water-dependent conductivity and excellent biocompatibility [45].

However, one of the most important issues displayed by these materials is connected to

the relatively low solubility and, hence, processability. Currently, this problem has been

overcome by employing different strategies, and now it is possible to commercially obtain

soluble eumelanin samples synthesized from tyrosine with hydrogen peroxide [45]. From

the application point of view, the affinity of eumelanin to metal ions has been exploited to

fabricate bioelectronic devices. For example, it has been shown that the addition of copper

ions can modulate the eumelanin conductivity up to four orders of magnitude. A solid-

state OECT based on this eumelanin/copper composite also demonstrated a performance

enhancement with metal chelation [45].

4.4.3 Photoswitches

Another important class of optical transducers is represented by molecular photo­

switches that permit modulate cell signaling via a photomechanical effect. In this case,

the transduction mechanism originates from the spatial rearrangement of the con­

formational state upon photoexcitation, which translates into a marked change of the

absorption spectrum. It is important to note that such a mechanism is inherently bio­

mimetic as it reproduces the initial fate of the retinal, the chromophore in the retina

photoreceptors that is responsible for light sensitivities. Photoswitches are largely em­

ployed in several technologies. Beyond classical applications in optoelectronics and data

storage, the use of photoswitches to regulate physiological signaling attracted a lot of

attention in the last couple of decades. For instance, tethered azobenzenes have been

covalently linked to the plasma membrane or ion channels, allowing modulation of

the cell potential dynamics in a light-dependent fashion [46]. Alternatively, the non-

covalent affinity of the molecules can be exploited to selected bio-target. In the seminal

works of Fujiwara and Yonezawa, an aliphatic amphiphilic azobenzene derivative was

employed to change the capacitance of black lipid membranes in response to prolonged

ultraviolet illumination [47,48]. However, non-covalent optostimulation of neurons with

photoswitches has not been achieved until recently. In this regard, our group has pro­

posed new amphiphilic azobenzenes that dwell in the plasma membrane without the

need for covalent attachment, inducing light-evoked action potential firing both in vitro

and in vivo [49]. The optomechanical stimulation mechanism stems from the trans → cis

photoreaction of azobenzenes: in the dark, the trans isomer can undergo dimerization

causing a thinning of the membrane and an increase of its electrical capacitance, while

illumination triggers the formation of a stable population of cis isomers and, thus, to the

disruption of the dimers leading to a restoration of membrane thickness and capacitance

(Figure 4.6) [50–52].

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