NOVELTY - Making (M1) at least one metal nanoparticles involves providing a solution comprising a first metal ion; providing a plant extract that comprises a reducing agent, a polyphenol, caffeine, and/or a natural solvent or surfactant; and combining the first metal ion solution and the plant extract to produce metal nanoparticles.    USE - For making metal nanoparticles selected from iron nanoparticles, manganese nanoparticles, silver nanoparticles, palladium nanoparticles, gold nanoparticles, platinum nanoparticles, copper nanoparticles and/or indium nanoparticles useful in an application selected from soil and groundwater remediation, water and wastewater treatment, air pollution treatment, medical diagnostic testing, medical materials, targeted drug delivery, cancer cell and DNA hypothermic inactivation, biological agent inactivation, toxicity reduction, catalysis of chemical synthesis reactions, pollution control or monitoring devices, fuel cells, and electronics; for reducing the concentration of a contaminant e.g. perchlorate, nitrate, a heavy metal, a heavy metal compound, Hg2+, Ni2+, Ag+, Cd2+, Cr2O72-, AsO43- in a medium selected from a biologically contaminated material, soil, groundwater, water, wastewater and/or air; in a device selected from medical diagnostic test, a medical device (e.g. bandage), a targeted drug delivery vehicle, a chemical synthesis system, a pollution control or monitoring device, a fuel cell, and electronic device (claimed).    ADVANTAGE - The concentrated plant extract produces greater yield of metal nanoparticles than produced using plant extract that has not been concentrated. The metal nanoparticles are concentrated and substantially free of liquid. The green methods of making metal nanoparticles reduce or eliminate the use and generation of substances hazardous to human health and the environment. The tea and coffee extracts have high water solubility and low toxicity and are biodegradable. The nanoparticles can be produced without a separate surfactant, capping agent, or template. The method allows preparation of nanoparticles at room temperatures and pressures, without the use of toxic or hazardous chemicals or the production of wastes containing toxic or hazardous chemicals. The synthesis of metal nanoparticles, such as zero valent iron nanoparticles, with natural resources, can avoid generating hazardous waste and thus can reduce environmental risk. The method for making metal nanoparticles with plant-based extracts can be easier and safer than conventional methods of making metal nanoparticles. The use of environmentally benign and renewable materials in the production of metal nanoparticles is important for pharmaceutical and biomedical applications. The concentration process can produce a higher concentration of plant polyphenols, enabling a high concentration of dissolved metal to be used to make higher concentrations of nanometal particles. The metal nanoparticles have a high degree of dispersibility. The metal nanoparticles can be much easier to handle because they are less susceptible to aggregation than are metal nanoparticles prepared using other methods. The metal nanoparticles are formed at a rate of at least 0.002 (preferably at least 0.01, especially at least 0.1, particularly at least 0.5) mol/l/minute.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a composition (C1) comprising a metal nanoparticle prepared by the method (M1);    (2) a composition (C2) comprising zero valent metal nanoparticles and a plant extract or component of the plant extract that comprises a reducing agent, a polyphenol, caffeine, and/or a natural solvent or surfactant;    (3) reducing the concentration of a contaminant in a medium involving combining the metal nanoparticle with the medium; and allowing the metal nanoparticle to reduce or stimulate biological reduction of the contaminant to reduce its concentration;    (4) determining an optimal amount of dissolved metal ion to add to a plant extract solution in synthesizing metal nanoparticles involving measuring (2,2-diphenyl-1-picrylhydrazyl) (DPPH) absorbance for a series of plant extracts with a range of concentrations, measuring DPPH absorbance for a series of nanoparticle preparations produced from the plant extracts and a metal ion solution, comparing the DPPH absorbance of the aqueous solutions of the series of plant extracts and the series of nanoparticle preparations to determine the net consumption of DPPH, and determining the optimal ratio of dissolved metal ions to plant extract; and    (5) method (M2) of making at least one metal nanoparticles involving providing a dissolved metal ion, providing a plant derivative selected from carboxy methyl cellulose and a plant extract, where the plant extract comprises a reducing agent, a polyphenol, caffeine, and/or a natural solvent or surfactant, and combining the dissolved metal ion and the plant derivative to produce the metal nanoparticles.