Table 1.

Methods and results of the selected technologies for improving the sensitivity of the protein assays

Technique	Sample	Detection methods	LODs	Multiplexed?	Assay time	Main instrument or reagent	Pros/cons
Optically enhanced protein microarray [25]	Diluted serum	Amperometry	10 fg/mL	Y	Hours	Gold compact discs	Pros: High sensitivity, simplicity Cons: Requires gold discs
dELISA with micro-beads [40]	Serum	Fluorescence	~10–20 enzyme-labeled complexes in 100 mL of sample	Y	Hours	dELISA setup	Pros: High sensitivity, no need for complex equipment Cons: Requires multiple repetitive steps
Hydrogel microparticle immunoassay [43]	Serum	Fluorescence	17. 7 pg/mL	Y	Hours	Microfluidic chip	Pros: High sensitivity Cons: Multiple assay steps required
CNT-assembly nanosensor [46]	Serum	Dielectrophoretic	100 fM	N	Hours	CNTs	Pros: Low limit of detection Cons: Requiring access to carbon nanotubes implemented on a gold electrode
Beads-in-hydrogel droplet microarray [48]	Serum	Fluorescence	<100 pg/mL	Y	Hours	Microarray scanner	Pros: Multiplexing capacity, low cost, and high sensitivity Cons: Requiring handling with care during the assay as hydrogel droplets are fragile
Microfluidic plasma separation and ELISA device [55]	Whole blood	Absorbance-based optical detection	0.7 ng/mL	Y	22 min	Microfluidic ELISA device	Pros: Analyzing whole blood in a short amount of time, high sensitivity, and requiring small sample volume Cons: required microfabrication
QDs-based protein detection [59]	Cell media	Luminescence	180 attomolar	Y	Hours	A spectrophotometer	Pros: achieved single-cell level Cons: required microfabrication to isolate single cells
Proximity ligation assay [64]	Plasma	Quantitative real-time PCR (q-PCR).	3.1 pg/mL	Y	Mins-hours	PLA probes, PCR setup	Pros: High sensitivity and specify Cons: Multiple steps in the manufacturing and assay procedure required
Fiber optic SPR probe for immunoassay [67]	Serum	Surface plasmon resonance (SPR)	0.1 μg/mL	N	Mins–hours	SPR biosensors	Pros: High sensitivity, simplicity, miniaturization, and low cost Cons: Required microfabrication process
Gold and silver nanoboxes with SERS [3]	Serum	SERS with nanotags	0.68 pg/mL	Y	Mins–hours	UV−vis spectrophotometer	Pros: High sensitivity and multiplexity Cons: Required access to gold-silver nanoboxes
Affinity-MS assays [70]	Serum/plasma	MS	2 μg of total starting protein from digested cell culture lysates	Y	Hours	MALDI mass spectrometer	Pros: multiplexed, sensitive, specific Cons: Required a high-end MALDI mass spectrometer