New insight into naturally occurring nanosilver: Role of plant root exudates

Transcription

1 New insight into naturally occurring nanosilver: Role of plant root exudates Huiyuan Guo Ph.D. candidate Advisor: Prof. Baoshan Xing Environmental soil and chemistry program University of Massachusetts Amherst /6/7 Los Angeles, CA

2 Number of Products Background: sources of nanosilver (nag) Engineered nag from consumer products Major Nanomaterials ( (Guo et al. Environ. Sci. Technol)

3 Background: sources of nanosilver (nag) Naturally formed nag Reducing sugars EPS from microorganims (e.g. E.coli) nag Oxidation& dissolution Ag + Reduction nag EPS: extracellular polymeric substances

4 Root exudates (RE) Ubiquitous in soil and aquatic environments; A cocktail of organic substances and inorganic ions; Unknown role in the formation and stability of naturally occurring metal nanoparticles. Research goals: Root exudates Reduction + Ag + nag Investigate if RE own the ability to transform Ag + to nag If yes, identify the key factors that control the reduction of silver ions by RE and figure out the underlying mechanisms

5 Experimental design Reduction of Ag + by root exudates (RE) from wheat plants RE collection and characterization Greenhouse: with 6h/8h (light/dark) cycle, light intensity ~ lux. RE were collected from ~ week old of live wheat roots after h incubation in water. TOC: ± mg/l, ph 6.7±.. GC-MS Interaction of RE with Ag + The extracted RE interacted with Ag + ( mm) in a chamber room with light intensity of 888 lux. Characterization and determination of the formed particles UV-vis spectroscopy Surface-enhanced Raman spectroscopy (SERS) SEM-EDS XRD

6 Absorbance Absorbance h. h h h h h after well mixing. nm h. h h h h h Time (h) Root exudates can reduce Ag + to nag. The AgNP concentration increased in of irradiation and leveled off thereafter.

7 Raman intensity (A. U.) Surface-enhanced Raman spectroscopy (SERS) data Indicator: ferbam RE-formed particles+ferbam Standard nag (6 nm, citrate)+ferbam Raman shift (cm - ) SERS data confirmed the formation of nag.

8 Potential mechanisms Superoxide anion radical (O - ) Electron shuttling within Ag + - organic complexes Enzymes or other proteins Reducing sugars

9 Absorbance (a.u.) Absorbance (a.u.) Absorbance (a. u.). Superoxide anion radical (O - ). h Air No purge N No SOD SOD SOD6 SOD effect- nm h h. Ag ions Ag ions+sod U/mL Ag ions+sod6 U/mL. t/h. No purge Purge with air. Purge with N. 6 8 No purge Purge with Air Purge with N Superoxide anion radical is not responsible for reduction of Ag +.

10 Absorbance at nm. Electron shuttling within Ag + -organic complexes [Na + ]/mm: mm mm mm Time/h Competing cation didn t affect the initial reaction rate but rather resulted in the particle aggregation because of the high ionic strength. The reduction of Ag + is not attributed to the ligand-to-metal charge transfer.

11 . Enzymes or other proteins boiled RE Unboiled RE Boiled. 8 Unboiled Enzymes or other proteins in root exudates didn t contribute to the transformation of Ag + to nag.

12 . Reducing sugars Reducing sugars (Based on GC-MS data) Relative abundance Reduction of Ag + D-mannose.% No D-fructose.6% No α-d-lactose.% No D-cellobiose 8.9% No Mixture of sugars 79.% No

13 RE fractions based on molecular weight > kda h. h. h h h - kda h. h. h h h - kda h. h. h h h kda h. h h h h nm nm - kda h. h h h h RE (.6 ppm Cl ions) h. h 6 nm h nm h h The fraction of - kda in root exudates plays a critical role in the reduction of silver ions, likely due to the presence of Cl ions in the - kda. Without the > kda fraction, the formed particles are less stable.

14 Intensity (cps) SEM EDS EDS and XRD data demonstrate that both nag() and nagcl are present in the formed particles. XRD AgCl Ag θ (º) 6 8

15 > kda, no Cl- h. h h h h (a) (c) < kda h. h h h h (b) > kda+.6 ppm Cl ions (d) RE (.6 ppm Cl ions) h. h h h h h. h h h h

16 >kda+cl >kda <kda RE h, d in fridge Adding Cl ions into the fraction of > kda leads to the formation of Ag-containing particles. Both > kda and - kda are responsible for the stabilization of the formed Ag-containing particles.

17 Proposed Mechanism hv ClO - h + Cl - Redox cycling Ag + /RE nagcl/re nag()/re e - Ag + x Ag() agg Future work. Influence of the Ag/Cl ratio in the system on the reduction pattern of Ag +.. Characterize the structure difference using scanning electron microscopy at different ratios of Ag/Cl.. Quantify the fraction of AgCl and Ag() in the formed particles.

18 Conclusions. Root exudates possess the ability to reduce Ag +.. The reducing strength doesn t derive from superoxide radicals, Ag + - organic complexes, enzymes or other proteins, or reducing sugars.. Cl - in root exudates plays an important function in converting Ag + because of the electrons generated by AgCl upon light exposure.. Both fractions of - kda and > kda are responsible for the stabilization of the formed nagcl and nag particles. Significance: A new pathway leading to natural formation of silver-containing nanoparticles. Impact of environmental cycling on the fate and toxicity of silver ions and silver-containing nanoparticles.

19 Acknowledgements Advisor: Dr. Baoshan Xing Committee members: Dr. Jason C. White Dr. Lili He Dr. Wesley R. Autio Lab members: Dr. Feng Han My Huynh Lauren Thistle USDA NIFA Hatch Program (MAS7) USDA NIFA (-677-7) BARD (IS-96-6R) Thanks for your attention!

20 SI

21 Background Environmental contamination of silver ions: 6 µg/l near photographic manufacturing waste discharges; µg/l in treated photoprocessing wastewaters; mg/kg in certain soils; and mg/kg in river sediments. Environmental toxicity of silver ions: Lethal concentrations for sensitive aquatic plants, invertebrates, and teleosts: - µg/l. Adverse effects occur on development of trout at concentrations as low as.7 µg/l and on phytoplankton species composition and succession at.-.6 µg/l. --WHO: Silver and Silver Compounds: Environmental Aspects Reduction of silver ions: Microorganims (e.g. E.coli) Ag + Reduction nag

22 . 6.7 mg/l Ag + are more toxic than the same levels of AgNPs. (Ma et al. ) The ability of plant root exudates to transform more toxic Ag + to less toxic AgNPs provides one possible pathway for plants defending the stress caused by heavy metal ions. Meanwhile, the alteration of silver species by plants may change their fate and toxicity of heavy metal ions in other living organisms.

23 Frequency (%) SEM Size range (nm) Size characterization Average diameter nm < nm % > nm 9%

24 h (a) (b) (c) (b) Ag/Cl=: RE: 7.8 ppm Ag/.6 ppm Cl> nm h. h 6 nm h nm nm h nm nm h 6 nm nm (a) (c) Ag/Cl=: h. h h h h h. h h h h The ratio of Ag/Cl in the system can modify the reduction pattern of Ag +.

25 HNO NH OH Original HNO NH OH Original NH OH RE (a) µl µl µl Ag/Cl=/ (c) µl µl 6 µl µl.8.6 HNO µl µl µl 8 µl 8 µl 8 µl 68 µl 88 µl The fraction of Ag() and AgCl can be observed separately by using NH OH and HNO. (b) (d) µl. µl. µl 7. µl. µl 7. µl 6. µl. µl 8. µl h later