Dear friend Va Ab

Thank you for sharing your challenge regarding the concentration of nanoparticles and the removal of excess surfactant. This is indeed a nuanced problem, and I appreciate the opportunity to think through potential solutions with you.

To begin, when it comes to concentrating nanoparticles without ultracentrifugation, gentle evaporation techniques—such as rotary evaporation under reduced pressure or controlled heating—can be effective in reducing solvent volume while minimizing aggregation. If your setup allows, these methods might help you achieve the desired concentration without specialized equipment.

For surfactant removal, selective precipitation could be promising. Adjusting parameters like pH or temperature may cause the surfactant to precipitate or separate from the nanoparticle suspension, allowing for easier removal. Alternatively, exploring filtration materials with tailored pore sizes or surface chemistries could enable selective trapping of surfactant molecules while letting nanoparticles pass through.

If you’re open to other chemical separation principles, methods such as dialysis, solvent exchange, or even exploiting differences in solubility could be worth investigating. Each approach has its own considerations, but by leveraging the unique properties of your system, you may find a solution that fits your lab’s capabilities.

Would you be interested in discussing any of these strategies further, or perhaps sharing more details about your current setup? I’d be glad to help brainstorm or review any protocols you’re considering.

Best regards,

Kaushik

Dear Kaushik Shandilya,

Thank you for your thoughtful response.

I'm preparing SLNs using the solvent injection method. Specifically, I dissolve stearic acid in a 1 mL mixture of acetone/ethanol, then inject it into 5 mL of Pluronic F127 aqueous solution, followed by cooling on an ice bath to promote nanoparticle formation.

Regarding your evaporation suggestion, I've considered gentle vacuum evaporation at room temperature, but without a rotary evaporator, it takes quite a while (potentially days), and I'm concerned about aggregation during that time.

On selective precipitation, that sounds viable. I've tried adjusting pH down to 1.2 and adding salts, but not all particles settle even at 15,000g centrifugation. Is there a way to induce flocculation more effectively, perhaps with specific agents or conditions tailored to stearic acid or pluronic, to allow sedimentation with a standard centrifuge and then wash the pellet?

If you have protocols or references in mind, that would be great

You can do this without an ultracentrifuge, centrifugal filters, or dialysis by combining size-exclusion chromatography (SEC) to wash out free Pluronic F127 and gentle vacuum concentration (or straight lyophilization with cryoprotectants) to concentrate. Below are three practical routes that work for stearic-acid/F127 SLNs.

Route 1 — Size-Exclusion Chromatography (best balance of purity/yield)

Goal: Separate 100–200 nm SLNs from free F127 micelles/unimers (≲10–20 nm).

What you need

• Gravity SEC column packed with Sepharose CL-2B / CL-4B or Sephacryl S-500 HR (all exclude 90% reduction of free surfactant in one pass. A second short pass gives extra cleanup if needed.

Concentration (post-SEC)

• Rotary evaporation / gentle vacuum at ≤25 °C to reduce volume (avoid foam; slow speed; no antifoam if you’ll lyophilize).
• Or proceed directly to lyophilization (see cryoprotectants below).

Route 2 — Adsorptive removal of free surfactant (fast, no membranes)

Goal: Scavenge free Pluronic without stripping the stabilizing corona from SLNs.

Materials

• Hydrophobic adsorbent beads: Bio-Beads SM-2/SM-4 or Amberlite XAD-2/7/16 (washed thoroughly: methanol → water).

Protocol

Cautions

• Over-adsorption can strip the corona and destabilize SLNs. Aim to leave ~0.01–0.05% w/v F127 residual to maintain colloidal stability.
• Validate by zeta potential / DLS and a simple surfactant assay (see QC).

Concentration

• As above: vacuum evaporation ≤25 °C or straight lyophilization.

Route 3 — Aqueous SEC “desalting” columns (plug-and-play)

If you don’t want to pack a column: use pre-packed gravity SEC desalting columns (long-bed, e.g., for EVs). They work similarly to Route 1; just follow the vendor’s gravity protocol. Two sequential columns often suffice to bring free F127 near background.

Lyophilization (to concentrate and dry)

SLNs typically tolerate freeze-drying well with proper protectants:

• Cryoprotectant/s: Trehalose 5–10% w/v (+ optional mannitol 1–2% w/v as bulking agent).
• Optional corona stabilizer in reconstitution: 0.01–0.05% F127 (keeps redispersibility high even after you removed most free F127).
• Freeze rapidly (–80 °C or liquid N₂), primary dry at ≤–20 °C shelf, then secondary at 10–20 °C to low residual moisture.
• Reconstitute in water or buffer; confirm size/PDI matches pre-freeze.

Quality control (quick and cheap)

• DLS size/PDI before/after each step.
• Zeta potential (optional) to monitor surface changes.
• Assay residual F127 (pick one): Iodine–PEO complex (absorbance ~480 nm) for PEO-containing surfactants such as Pluronics. Phenol–sulfuric acid assay (total polyether signal). Surface tension: tensiometry drop in γ indicates excess surfactant.
• Stability stress: 1 week at 4 °C & 25 °C; check DLS/turbidity.

What to avoid (with stearic-acid SLNs)

• Salt-induced flocculation to “wash”: often irreversible aggregation.
• Organic antisolvent additions (ethanol/IPA) to precipitate surfactant: risk dissolving lipids and collapsing SLNs.
• Heating above room temperature during concentration: stearic acid softens—avoid coalescence.

Minimal kit version (no special columns)