The oxidation of the AgNP labels by MnO4
– (previously added to the sensor) yielded soluble Ag+
that could be electrodeposited on the working electrode and detected by anodic strip voltammetry.
The paper test combined a simple origami assembly with microfluidic hollow channels to accommodate
micrometer scale particles. Additionally, a slip layer was used to control the timing of incubation
steps whereas a magnet allowed concentrating the MB-conjugated DNA sandwich on the surface of
the working electrode. The combination of the MBs and AgNPs provided signal amplification over
250,000 fold and a detection limit of 85 pM [40].
2.4.2. Inkjet-printing
Along with other printing methods, inkjet-printing has become one of the most promising
techniques capable of manufacturing cost-effective, disposable biosensors. It provides significant
advantages over other related fabrication technologies, such as screen-printing, because of the
versatility, the high precision and resolution of the printed patterns and importantly the ability
to deposit very small volumes of ink and/or biomolecules (picolitres) in a rapid and low-cost
procedure [51]. Although it is expected to grow in importance in the field of electrochemical paper
sensing devices based on biological recognition, so far it has only been applied in a couple of works.
The hydrophobic pattern materials and respective methods of preparation, as well as the nature
of the electrode conductive inks of paper-based inkjet-printed biosensors, are identical to those
previously described for screen-printed bioelectrodes. Similar interface modification strategies have
also been applied. For example, in the glucose biosensor proposed by Määttänen et al., the enzyme
(GOx) was entrapped in a poly-3,4-ethylenedioxythiophene (PEDOT) film electropolymerized on the
working electrode’s surface [28]. The biosensor was developed on an inkjet-printed platform based on
multilayer-coated recyclable paper. The electrodes consisted of printed colloidal gold (working and
counter) and Ag (reference) strips, the latter being covered with an additional layer of electrochemically
deposited Ag/AgCl. The low-cost device could perform selective glucose sensing by amperometry
with a linear response range from 0.1 up to 5 mM [28].