**2. Porous silicon biosensors: construction and transduction principles**

The aim of a biosensor is to produce either discrete or continuous signals, which are propor‐ tional to a single analyte or a related group of analytes [14]. Because of its particular proper‐ ties, the p-Si can be used as a transducer to convert this analytes into an optical or electrical signal [1]. Its large surface area enables an effective capture of the biological analytes al‐ though such a large surface area also implies high reactivity with the enviroment. This can cause the degradation of the biosensor and/or possible false positives. For this reason, stabili‐ zation of the p-Si surface via an appropriate surface chemistry is a required step for obtain‐ ing a succesful biosensor [15]. The surface chemistry should be designed in such a way as to obtain the desired effects, and yet still displaying bioactivity [16]. Also the binding affinity with the studied analytes must be taken into account [15, 17]. Some common techniques to function‐ alize p-Si include: oxidation [18, 19, 20], silanization [1, 15, 21, 22, 23, 24, 25], hydrosilylation of alkenes and alkynes [27, 28], radiation [29], and other chemical approaches [15, 16].

A proper pore-size distribution helps to achieve an efficient biosensor; p-Si fabricate from p+ and n+ -type silicon substrates is mesoporous, and suitable for immobilisation of biomacro‐ molecules, while p-Si from p-type substrates, whose pore diameter is of the order of a few nm, is suitable only for very small molecules [12]. Macroporous p-Si from n-type substrates may accommodate larger molecules [12].

Once the appropriate chemical functionalization and porous distribution size is obtained, the challenge then becomes transducing the recognition of the biological analytes into a measur‐ able signal. The requirements for efficient transduction are precision (same response to the same stimuli: repeatability) and accuracy (indicating magnitude value as close as possible to the real magnitude of the stimulus to be sensed: minimum absolute error spread) [30].

In general, the most common transduction techniques include piezoresistance, piezoelectric‐ ity, capacitive, resistive, tunneling, thermoelectricity, optical and radiation-based techni‐ ques, and electrochemical methods [31]. In the case of p-Si biosensors the most frequently used tecniques are optical and electrical/ electrochemical [1]. Here, we classified the p-Si bio‐ sensors depending on the transducing mechanism in optical and electrochemical contexts. Their characteristics and some related examples are detailed in the following sections.
