Point-of-Care Diagnostics - Holographic Sensors - page 15

1.2.6 Fluorescence

This detection mechanism was first demonstrated on paper microzone plates.

Although paper-based plates are known [ 150 - 153 ], they have recently been

suggested for quantitative fluorescence measurements. These paper plates require

low sample volumes; 12.5 pmol of

fl

fl fluorescein isothiocyanate-labeled bovine serum

albumin generated a relative 850 a.u. [ 133 ]. Although

concentration sensitivity and mass sensitivity were comparable with plastic plates,
the average relative standard deviation for the replicates of all concentrations was
higher. This limitation was attributed to light scattering on the cellulose

fl fluorescence of 2,700 ±

bres and

the in uence of the index of refraction between cellulose and air. Although paper-

based plates were suggested as alternatives to conventional plastic multiwell plates,
their interpretation required a microplate reader. Other notable studies adopting
fl

fl

fluorescent sensing included paper strips comprising DNA-conjugated microgels
(MG) for DNA detection [ 134 ]. Sensing DNA was accomplished by: (i) targeting
DNA promoted ligation of a DNA primer to the MG-bound DNA, (ii) rolling circle
ampli cation (RCA) between the primer and a circle DNA, and (iii) hybridisation

of the RCA products and a

uorescent DNA probe. Another study reported a
portable device for the growth of bacteria or the ampli

fl

cation of bacteriophages.

A fluorescent mCherry reporter was used to quantify the growth of bacteria and the

concentration of arabinose [ 87 ] (Fig. 1.2 e). Another paper-based platform involving
fl

fl

fluorescent sensing described the use of non-enzymatic nucleic acid circuits based
on strand exchange reactions for the detection of target sequences [ 135 ]. Overall,
although

fluorescence sensing brings new capabilities to point-of-care diagnostics,
the feasibility requires reduction in cost and miniaturisation of

fl

fl uorescence readers.

1.2.7 Genetically-Engineered Cells

Bacterial quorum signalling (QS) molecules, N-acyl homoserine lactones (AHLs),
have been used as a sensing mechanism [ 140 ]. The bacterial cell-based sensing
system comprised two main components: (i) AHL-mediated QS regulatory system
as recognition elements, and (ii) -galactosidase as the reporter enzyme. The

bacterial cells were inoculated on paper by liquid drying. The paper strip biosensor
detected low concentrations (0.1 nM) of AHLs in saliva. The advent of synthetic
biology will accelerate the development of whole-cell based biosensors.

β

1.3 Next Generation Diagnostics

To expand the current capabilities of point-of-care diagnostics, the materials,
sensors and readout devices need to evolve. Extending beyond strip tests and
lateral-

fl ow design to multiplexed miniaturised assay con gurations will put the