Block 3 - Wet Chemical Synthesis & Silver/Gold NPs

What is wet-chemical synthesis?

A bottom-up approach where NPs are formed form atomic/molecular precursors through chemical reactions in a liquid medium.

What is the general sequence of wet-chemical synthesis?

Metal ions (metal salt) → reduction → neutral metal atoms → nucleation → growth → NPs

What are the four major components of wet-chemical synthesis?

Metal Salt

Reducing Agent

Stabilizing Agent

Capping Agent

What does the metal salt/precursor provide?

It provides the metal ions that will eventually be reduced to form NPs

What does a reducing agent do?

Donates/loses electrons to metal ions, causing the metal ions to be reduced to neutral atoms

During reduction, does the metal ion gain or lose electrons?

It gains electrons

What happens to Au3+ during reduction?

It gains electrons and is reduced to Au0

What is the basic gold reduction relationship?

Au3+ + electrons → Au0

What happens to the reducing agent during the reaction?

It loses/donates electrons and is therefore oxidized

What does a stabilizing agent do?

It helps prevent uncontrolled NP aggregation and maintains colloidal stability

Why do newly formed NPs tend to aggregate?

Their large exposed surface gives them high surface energy, creating a tendency to aggregate

HIGH SURFACE ENERGY

What is aggregation?

NP clumping or associating together

What does a capping agent do?

Interacts with the NP surface and helps regulate growth, size, shape, and surface properties

Can stabilizing agents and capping agents have overlapping functions?

Yes, a molecule can sometimes contribute to both surface control and stabilizatione

Easy way to memorize WCS?

Precursor provides → reducer converts → stabilizer separates → capper controls

What variables can affect NP formation?

• Precursor concentration

• Temperature

• Reaction time

• Reducing/stabilizing agents

• pH/solvent

• Mixing/agitation

Why are reaction conditions important in WCS?

Changing them can alter NP size, shape, composition,, and size distribution

What are important advantages of WCS?

Versatility, scalability, control over NP size, shape, composition, and surface properties

Why is surface functionalization useful for NPs?

Allows the NP surface to be modified for specific interactions or applications

What is the Citrate/Turkevich Method?

A common WCS method for producing colloidal gold NPs using sodium citrate

What metal ion is reduced in citrate method?

Au3+ ions

What does Au3+ become during citrate method?

Au0

What are two major roles of sodium citrate?

Acts as both the reducing and stabilizing agents

How does citrate act as reducing agent?

It donates electrons to gold salt ions, reducing them to Au0, forming gold NPs

How does citrate act as stabilizing agent?

It controls the aggregation of newly formed NPs, keeps them separated

What is the approximate mean AuNP diameter associated with the citrate method?

~15 nm

What general temperature condition is used for citrate synthesis?

Elevated temperature

What does elevated temperature help accomplish?

It facilitates the chemical reduction and formation of gold NPs

What is the citrate synthesis method?

Au3+ metal salt precursor → citrate reduction → Au0 → nucleation/growth → Gold NPs

What is seed mediated synthesis?

Synthesis method where preformed small NPs (seeds) provide sites for subsequent NP growth

What happens after gold seeds are produced in seed-mediated synthesis?

They are introduced to a growth solution containing additional gold precursor and appropriate reagents

Why are seeds useful?

They provide controlled nucleation/growth sites, helping control NP morphology

What type of gold NP is associated with seed-mediated growth?

Gold nanorods

What is seedless synthesis?

Synthesis method where nucleation occurs directly in the reaction mixture without preformed seeds

Main difference between seed-mediated and seedless synthesis?

Seed-mediated: preformed seeds used

Seedless synthesis: Particles nucleate directly in the reaction mixture

What is biological NP synthesis?

Using biological sources or compounds to facilitate formation of NP from metal ions

What biological sources can be used?

Plant extracts, microorganisms, biological molecules/derivatives

What is one major role of biological compounds during NP synthesis?

They can act as reducing agents, converting metal ions to neutral atoms

What additional role can biological compounds perform?

They can act as stabilizing/capping agents

What is the general biological synthesis sequence?

Biological source/biomolecules + metal ions → reduction → neutral atoms → NPs

Can biological synthesis be used for both gold and silver NPs?

Yes

What happens to Ag+ during silver NP synthesis?

Ag+ gains an electron (reduced) and becomes neutral

What happens after Ag0 atoms form?

They undergo nucleation and growth to form AgNPs

What is the basic silver reduction sequence?

Ag+ → electron gain/reduction → Ag0 → nucleation/growth → AgNPs

What important species can silver nanoparticles release?

Ag+ ions

How can AgNPs/Ag+ affect bacterial?

• They can cause membrane damage/disruption

• They interfere with respiration and DNA replication

Why does high SA:V ratio benefit AgNPs against bacteria?

High SA:V means more contact with the bacteria and silver NP

What happens if nanoparticle stabilization is inadequate?

The particles can undergo uncontrolled aggregation

Is all NP aggregation bad?

Not necessarily, controlled aggregation can intentionally produce optical changes for sensing applications

What is the distinction between aggregation during synthesis and aggregation in biosensors?

• Synthesis seeks to prevent unwanted aggregation

• Biosensors induce controlled aggregation to generate a measurable optical response