Table 4.

Comparison of optical methods.

Optical Methods
Target Analyte	Detection Principle	Sensing Element	Sensing Material	Substrate	LOD	Real Sample	Ref.
Cu(II) Ni(II) Cd(II) Cr(VI)	Colorimetric	Chemical compound	Sodium diethyldithiocarbamate Dimethylglyoxime Cadion Diphenylcarbazide	Paper	0.29 ppm 0.33 ppm 0.19 ppm 0.35 ppm	Distilled water Reservoir water Beach water	[138]
Hg2+	Colorimetric	Nanoparticles	Platinum nanoparticles and 3,3,5,5-tetramethylbenzidine	Paper	0.01 uM	Pond and tap water	[139]
Pb2+	Colorimetric	Functionalized nanoparticles	AuNP functionalized with 11-mercaptoundecanoic acid	PDMS	10 $\mathsf{\mu }$M	No	[140]
As(III)	Colorimetric	Functionalized nanoparticles	AuNP functionalized with $\alpha$-lipoic acid and thioguanine	Paper	1.0 ppb	No	[141]
As	Colorimetric	Functionalized nanoparticles	AuNP functionalized with $\alpha$-lipoic acid	Paper	Quality analysis	Bangladesh groundwater	[142]
As(III)	Colorimetric	Hach®EZ Arsenic Test Kit	Standard Gutzeit reaction reagents	Plastic	3 $\mathsf{\mu }$g/L	No	[143]
Cu2+ and Hg2+	Fluorescence	Quantum dots	CdTe quantum dots	Paper	0.035 $\mathsf{\mu }$g/L 0.056 $\mathsf{\mu }$g/L	Lake and sea water	[144]
As(III)	Fluorescence	Bioreporter cell	E. coli	PDMS	10 $\mathsf{\mu }$g/L	Tap water	[145]
As(III)	Fluorescence	Bioreporter cell	E. coli	PDMS	50 $\mathsf{\mu }$g/L	No	[146]
As(III)	SERS	Functionalized nanoparticles	AgNP functionalized with glutathione/4-mercaptopyridine	PDMS	0.67 ppb	Tap water Mineral water	[147]
As(IV)	CL	Chemical compound	Luminol and Vanadomolybdoarsenate heteropoly acid	PDMS	$8.9\times 10$−8 M	Tap water	[148]
Nitrate	Colorimetric	Chromogenic agent	Chromotropic acid and Sulphuric acid	PDMS	0.70 mg/L	Drinking water, freshwater, wastewater, and sea water	[149]
Nitrate	Colorimetric	Chromogenic agent	Griess reagent	Fiber	7 $\mathsf{\mu }$g/L	Lake water Tap water	[150]
Nitrate	Colorimetric	Chromogenic agent	Griess reagent	PMMA	0.0782 ppm	Tap water Bottled drinking water Home-filtered water	[151]
Nitrite and Nitrate	Colorimetric	Chromogenic agent	Griess reagent Zinc microparticles	Paper	1.0 $\mathsf{\mu }$M 19 $\mathsf{\mu }$M	Tap water and synthetic water	[152]
Nitrite and Nitrate	Colorimetric	Chromogenic agent	Griess reagent Imidazole buffer	PMMA	0.02 $\mathsf{\mu }$M 0.025 $\mathsf{\mu }$M	River water	[153]
Nitrite and Nitrate	Colorimetric	Chromogenic agent	Griess reagent Copper-activated cadmium column	PMMA	20 nM	Sea water	[154]
E. coli	PCR	Biological elements	Polyclonal antibodies	PMMA	6 CFU	Recreational lake water, waste water	[155]
E. coli	Fluorescence	Biological elements	Magnetic beads conjugated with antibodies	PDMS	??	Drinking water	[156]
E. coli	Fluorescence	Biological elements	Streptavidin-coated magnetic markers	PDMS	??		[157]
E. coli	Light scattering	Biological elements	Antibody-conjugated beads	Paper	10 CFU/mL	Field water	[158]
E. coli S. aureus	SPR	Biological elements	Au surface modified with MUA, EDC/NHS, Protein G and anti-LPS antibody	PMMA	??	No	[159]
Roravirus	Fluorescence		Graphene oxide	Glass	105 PFU/mL	No	[160]
Antimicrobial- resistant bacteria	Colorimetric	Chromogenic agent	Nitrocefin	Paper	10 mU/mL	Sewage water, river water	[161]

CL: Chemiluminescence; ??: Not specified; SPR: Surface plasmon resonance; MUA: 11-mercaptoundecanoic acid; LPS: Lipopolysaccharide; EDC/NHS: Ethyl-3-(3-dimethy-laminopropyl) carbodiimide hydrochloride/N-Hydrosuccinimide.