Funding from the National Science Foundation through the NUE in Engineering and the LEADER Consortium Programs is gratefully acknowledged.

1. Synthesis of Colloidal AgNPs
The reaction mechanism for the Creighton method (slightly modified, inexpensive)22 is described in great detail in the Supporting information of reference Pavel et.al together with the undesired hydrolysis side-reaction of NaBH4 at room temperature or higher.23
<ol>
	<li>Clean all glassware for 12-24 hr in a 10% HNO3 bath, then for 4-12 hr in a 1.25 M NaOH in 40% ethanol bath, and finally autoclave. Glassware should ...

<ol>
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P., Nickerson, C., Franzen, S., Feldheim, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Purification+of+Molecularly+Bridged+Metal+Nanoparticle+Arrays+by+Centrifugation+and+Size+Exclusion+Chromatography.">Purification of Molecularly Bridged Metal Nanoparticle Arrays by Centrifugation and Size Exclusion Chromatography.</a> Anal. Chem. 73, 5758-5761 (2001).</li><li>Hossain, M. K., Kitahama, Y., Huang, G. 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N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemical+catalysis+by+colloids+and+clusters.">Chemical catalysis by colloids and clusters.</a> Chem. Rev. 93, 2693-2730 (1993).</li><li>Li, Y., Wu, Y., Ong, B. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Facile+Synthesis+of+Silver+Nanoparticles+Useful+for+Fabrication+of+High-Conductivity+Elements+for+Printed+Electronics.">Facile Synthesis of Silver Nanoparticles Useful for Fabrication of High-Conductivity Elements for Printed Electronics.</a> J. Am. Chem. Soc. 127, 3266-3267 (2005).</li><li>Sun, Y., Xia, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape-Controlled+Synthesis+of+Gold+and+Silver+Nanoparticles.">Shape-Controlled Synthesis of Gold and Silver Nanoparticles.</a> Science. 298, 2176-2179 (2002).</li><li>Han, X., Zhao, B., Ozaki, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-enhanced+Raman+scattering+for+protein+detection.">Surface-enhanced Raman scattering for protein detection.</a> Analytical and Bioanalytical Chemistry. 394, 1719-1727 (2009).</li><li>Pavel, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Label-Free+SERS+Detection+of+Small+Proteins+Modified+to+Act+as+Bifunctional+Linkers.">Label-Free SERS Detection of Small Proteins Modified to Act as Bifunctional Linkers.</a> J. 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UV-Vis Absorption Spectrophotometry and Raman Spectroscopy of Colloidal AgNPs
It is well known that the number of surface plasmon resonance peaks in the absorption spectrum of a colloid decreases as the symmetry of the AgNPs increases. Additionally, AgNP aggregation leads to the appearance of broader or red-shifted peaks.25,26 The presence of a single, sharp and symmetrical SPR peak at 394 nm is indicative of small, spherical AgNPs of moderate aggregation and size distribution.
<...

<table border="1">
 <tr>
 <td>Silver nitrate 			(AgNO3)</td>
 <td>Acros Organics 			Inc.</td>
 <td>CAS: 7761-88-8</td>
 <td></td>
 </tr>
 <tr>
 <td>Sodium 			borohydride (NaBH4)</td>
 <td>Acros Organics 			Inc.</td>
 <td>CAS: 			16940-66-2</td>
 <td></td>
 </tr>
 <tr>
 <td>Nitric acid 			(HNO3, 			Optima)</td>
 <td>Fisher 			Scientific Inc.</td>
 <td>A467-1</td>
 <td>Trace 			metal grade for ICP 			analysis</td>
 </tr>
 <tr>
 <td>10,000 &mu;g 			ml-1 			silver standard, EnviroConcentrate</td>
 <td>Ultra 			Scientific</td>
 <td>US-IAA-047</td>
 <td></td>
 </tr>
 <tr>
 <td>KrosFlo 			Research IIi Tangential Flow Filtration System</td>
 <td>Spectrum 			Laboratories Inc.</td>
 <td><a href="http://www.spectrumlabs.com/pshop/RequestPrice/index.html?LiFrom=%2Ffiltration%2FKR2SystemO.html;FrName=Research+IIi+System;Pn=SYR2-U20-01N;" target="_blank">SYR2-U20-01N</a></td>
 <td></td>
 </tr>
 <tr>
 <td>0.05 			&mu;m PS (0.5 mm) 460 cm2</td>
 <td>Spectrum 			Laboratories Inc.</td>
 <td><a href="http://www.spectrumlabs.com/filtration/MidiKros.html?Pn=X30S-900-02N;" target="_blank">X30S-900-02N</a></td>
 <td></td>
 </tr>
 <tr>
 <td>Midi 100 kD PS 			200 cm2</td>
 <td>Spectrum 			Laboratories Inc.</td>
 <td>X3-100S-901-02N</td>
 <td></td>
 </tr>
 <tr>
 <td>Micro100 kD PS 			20 cm2</td>
 <td>Spectrum 			Laboratories Inc.</td>
 <td>X1AB-300-10N</td>
 <td></td>
 </tr>
 <tr>
 <td>MasterFlex 			C-Flex tubing L/S Size 17 </td>
 <td>Cole-Palmer 			Instrument Co.</td>
 <td>06424-17</td>
 <td></td>
 </tr>
 <tr>
 <td>MasterFlex 			C-Flex tubing L/S Size 14 </td>
 <td>Cole-Palmer 			Instrument Co.</td>
 <td>06424-14</td>
 <td></td>
 </tr>
 <tr>
 <td>Cary 50 			UV-VIS-NIR spectrophotometer</td>
 <td>Varian Inc.</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>LabRam HR 800 			system</td>
 <td>Horiba Jobin 			Yvon Inc.</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Varian 710ES 			ICP-OES</td>
 <td>Varian Inc.</td>
 <td></td>
 <td></td>
 </tr>
</table>
Table 1. Specific reagents and equipment.

tangential flow ultrafiltration: a  &ldquo;green&rdquo; method for the size selection and concentration of colloidal silver nanoparticles

Nowadays, AgNPs are extensively used in the manufacture of consumer products,1 water disinfectants,2 therapeutics,1, 3 and biomedical devices4 due to their powerful antimicrobial properties.3-6 These nanoparticle applications are strongly influenced by the AgNP size and aggregation state. Many challenges exist in the controlled fabrication7 and size-based isolation4,8 of unfunctionalized, homogenous AgNPs that are free from chemically aggressive capping/stabilizing agents or organic solvents.7-13 Limitations emerge from the toxicity of reagents, high costs or reduced efficiency of the AgNP synthesis or isolation methods (e.g., centrifugation, size-dependent solubility, size-exclusion chromatography, etc.).10,14-18 To overcome this, we recently showed that TFU permits greater control over the size, concentration and aggregation state of Creighton AgNPs (300 ml of 15.3 &mu;g ml-1 down to 10 ml of 198.7 &mu;g ml-1) than conventional methods of isolation such as ultracentrifugation.19
TFU is a recirculation method commonly used for the weight-based isolation of proteins, viruses and cells.20,21 Briefly, the liquid sample is passed through a series of hollow fiber membranes with pore size ranging from 1,000 kD to 10 kD. Smaller suspended or dissolved constituents in the sample will pass through the porous barrier together with the solvent (filtrate), while the larger constituents are retained (retentate). TFU may be considered a "green" method as it neither damages the sample nor requires additional solvent to eliminate toxic excess reagents and byproducts. Furthermore, TFU may be applied to a large variety of nanoparticles as both hydrophobic and hydrophilic filters are available.
The two main objectives of this study were: 1) to illustrate the experimental aspects of the TFU approach through an invited video experience and 2) to demonstrate the feasibility of the TFU method for larger volumes of colloidal nanoparticles and smaller volumes of retentate. First, unfuctionalized AgNPs (4 L, 15.2 &mu;g ml-1) were synthesized using the well-established Creighton method22,23 by the reduction of AgNO3 with NaBH4. AgNP polydispersity was then minimized via a 3-step TFU using a 50-nm filter (460 cm2) to remove AgNPs and AgNP-aggregates larger than 50 nm, followed by two 100-kD (200 cm2 and 20 cm2) filters to concentrate the AgNPs. Representative samples were characterized using transmission electron microscopy, UV-Vis absorption spectrophotometry, Raman spectroscopy, and inductively coupled plasma optical emission spectroscopy. The final retentate consisted of highly concentrated (4 ml, 8,539.9 &mu;g ml-1) yet lowly aggregated and homogeneous AgNPs of 1-20 nm in diameter. This corresponds to a silver concentration yield of about 62%.

Watch this Scientific Journal Video about Tangential Flow Ultrafiltration: A  â€œGreenâ€ Method for the Size Selection and Concentration of Colloidal Silver Nanoparticles at JoVE.com

Tangential Flow Ultrafiltration: A  &ldquo;Green&rdquo; Method for the Size Selection and Concentration of Colloidal Silver Nanoparticles

Tangential flow ultrafiltration (TFU) is a recirculation method used for the weight-based separation of biosamples. TFU was adapted to size-select (1-20 nm diameter) and highly concentrate a large volume of polydisperse silver nanoparticles (4 L of 15.2 &mu;g ml-1 down to 4 ml of 8,539.9 &mu;g ml-1) with minimal aggregation.

Nowadays, AgNPs are extensively used in the manufacture of consumer products,1 water disinfectants,2 therapeutics,1, 3 and biomedical devices4 due to ...

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