3. VF-μPADs for Environmental Monitoring and Agriculture.

Specimen (volume, μL)	Fabrication method	Used materials	Sensing modality	Target analyte(s)	Detection limit	Ref.
Rainwater, Tap water (40 μL)	Stacking	Cellulose membrane	Colorimetric	Cu2+	Buffer-based: 1.7 mg/L
Groundwater, Stream water, (50 μL)	Stacking	Cellulose membrane	Electrochemical, Colorimetric	Pb2+, Cd2+, Cu2+	Buffer-based: 0.1 ng/L (Pb2+), 0.1 ng/L (Cd2+), 4.12 ng/L (Cu2+)
Contaminated water (NA)	Stacking	Cellulose membrane	Colorimetric	Ni2+, Cu2+, Hg2+, Cs+	Buffer-based: 0.55 ppm (Ni2+), 5.05 ppm (Cu2+), 0.188 ppm (Hg2+), 0.016 ppm (Cs+)
Drinking water, River water, Soil (3 μL)	Stacking	Cellulose membrane	Colorimetric	Hg2+	Buffer-based: 20 mg/L
Tap water, lake water (130 μL)	Stacking	Cellulose membrane	Colorimetric	Fe3+, Ni2+, Cr4+, Cu2+, Al3+, and Zn2+	Buffer-based: 0.2 mg/L (Fe3+), 0.3 mg/L (Ni2+), 0.1 mg/L (Cr4+), 0.03 mg/L (Cu2+), 0.08 mg/L (Al3+), 0.04 mg/L (Zn2+)
Lake water, Seawater (30 μL)	Stacking	Cellulose membrane	Colorimetric	Cd2+, Pb2+	Buffer-based: 0.245 μg/L (Cd2+), 0.335 μg/L (Pb2+)
Coastal water, Seawater (40 μL)	Stacking	Cellulose membrane	Colorimetric	Cu2+, Cd2+, Pb2+, Hg2+	Buffer-based: 0.25 μg/L (Cu2+), 0.17 μg/L (Cd2+), 0.23 μg/L (Pb2+), 0.36 μg/L (Hg2+)
PBS (3 μL)	Stacking	Cellulose membrane	Colorimetric	Pesticide	Buffer-based: 8.60 ppm
Tap water, Bottled water (2.5 mL)	Stacking	Cellulose membrane	Colorimetric	Hg2+	Buffer-based: 1.2 nM
River water (5 μL)	Origami/folding	Cellulose membrane	Electrochemical	Pesticide (paraoxon)	Buffer-based: 2 ppb (paraoxon)
Drinking water, River water (400 μL)	Origami/folding	Cellulose membrane	Colorimetric	Ni2+, Cu2+, Fe3+	Buffer-based: 2 ppm (Ni2+), 0.3 ppm (Cu2+), 1.1 ppm (Fe3+)
Liquid DMMP (20 μL)	Origami/folding	Cellulose membrane	Colorimetric	Dimethyl methylphosphonate (DMMP)	Semiquantitative
H2O, Concrete (10 μL)	Stacking	Cellulose membrane	Electrochemical	Chlorides	Buffer-based: 5 × 10–2 M
Airborne particulate matter (PM) from Fuzhou City, China (0.3 μL)	Stacking	Cellulose membrane	Colorimetric	Fe, Cu, Ni	Buffer-based: 16.6 ng (Fe), 5.1 ng (Cu), 9.9 ng (Ni)
Apple, Cucumber, Tomato (15 μL)	Stacking	Cellulose membrane	Colorimetric	Malathion (insecticide)	Buffer-based: 6 nmol/L
Fruit samples (50 μL)	Stacking	Cellulose membrane	Colorimetric	Phenolic compounds	Buffer-based: 0.5 mg/L
Wine samples (25 μL)	Stacking	Cellulose membrane	Colorimetric	Phenolic compounds	Buffer-based: 1.2 mg/L
Brassica pekinensis, Brassica chinensis (12 μL)	Stacking	Cellulose membrane	Colorimetric	Pesticides (profenofo, methomyl)	Buffer-based: 55 nM (profenofo), 34 nM (methomyl)
Whiskey samples (1 μL)	Origami/folding	Cellulose membrane	Colorimetric	Alcohol	Buffer-based: 2.10%
Seaweeds (35 μL)	Origami/folding	Cellulose membrane	Colorimetric	Iodine (I–, IO3–)	Buffer-based: 9.8 μM (I–), 0.6 μM (IO3–)