Seeing the Unseeable: How the ZE5 Cell Analyzer Is Bringing Nanoscale Particles Into Focus

For most of flow cytometry's history, “small” meant a cell, a few microns across, easy to light up and easy to gate. Anything smaller lived in an instrumentation blind spot. Bacterial membrane vesicles, extracellular vesicles, and lipid nanoparticles are all biologically important, all roughly a thousand times smaller than a cell, and all notoriously hard to see one at a time.

Three recent application notes from the Bio-Rad team put that blind spot to the test, using the ZE5 Cell Analyzer, fitted with its small particle detector (SPD), to look directly at bacterial membrane vesicles (MVs), extracellular vesicles (EVs), and lipid nanoparticles (LNPs) at true single-particle resolution.

Together, the studies make a simple case: you can finally characterize individual nanoscale particles instead of guessing at their average.

Why Bother With Single Particles?

MVs and EVs are tiny membrane-wrapped particles released by living cells, MVs from bacteria and EVs from human or animal cells, but both have outsized biological roles.

MVs move signals and nucleic acids between bacteria, deliver virulence factors, support immune evasion, and aid nutrient cycling; they are also being explored for vaccines and cancer therapies. EVs similarly carry mRNA, microRNA, proteins, and lipids between cells, and because they circulate in blood and reflect their source cells, they are attracting interest as liquid-biopsy markers and therapeutics.

LNPs are synthetic particles, typically 10–1,000 nanometers across, used to package drugs and therapeutic agents and protect them until delivery. However, traditional LNP quality control relies on bulk measurements, which show average encapsulation across billions of particles but not how many truly contain drug or how much cargo remains exposed on the surface.

Across all three, standard bulk assays obscure the particle-level heterogeneity that matters most.

Bacteria and Their Vesicles, Resolved

The first application note tackles a deceptively simple question: can you measure bacterial cells and the membrane vesicles they shed, in the same sample, at the same time? Using membrane dye FM 4-64 alongside the SPD's 405 nm forward-scatter channel, the team compared PBS alone, MVs alone, bacterial cells alone, and a combined sample. The populations separate cleanly: MVs sit dim-and-moderate on the plot, intact bacterial cells light up brighter with distinctly higher scatter, and mixing the two together simply overlays both signatures rather than blurring them (Figure 1).

Fig. 1.  FM 4-64 fluorescence vs. 405 nm forward scatter for a PBS control (A), MVs alone (B), bacterial cells alone (C), and a combined MV-plus-cells sample (D), acquired using the small particle detector.

The team then dug into what's actually inside the vesicles, co-staining MVs with a lipophilic membrane dye (DiR) and the DNA-binding dye Hoechst 33342. Comparing DiR signal to sizing beads suggested that most MVs measured between 50 and 240 nm in diameter, and 63.3% stained positive for double-stranded DNA, direct, particle-level evidence that MVs genuinely carry genetic cargo rather than just membrane fragments.

Watching EVs Light Up

The second note turns to EVs isolated from cultured cells via ultracentrifugation. The first challenge is confirming you're looking at real particles, not background noise. The team stained EVs with a lipophilic membrane dye (FI7510) and compared the signal against a blank PBS control and a set of sizing beads. The blank showed little signal; the EV sample lit up unmistakably, and cross-referencing against the beads suggested most particles fell in the 240–500 nm range, a size window conventional cytometers struggle to resolve cleanly from noise.

Fig. 2.  Forward scatter vs. FI7510 fluorescence for a blank PBS control (left), FI7510-stained EVs distinguishing small and large vesicle populations (center), and sizing beads from 160–500 nm for reference (right).

The more interesting result came next. The team asked whether these EVs actually display specific surface proteins, staining for two markers (protein X and protein Y) alongside antibody-only controls. The plots tell a satisfying story at a glance: antibody controls show almost nothing, while the real EV sample produces a clean, well-defined positive population for each marker individually. Both markers together resolve the population further: 27.7% of EVs were negative for both proteins, 15.3% expressed only protein X, 6.2% expressed only protein Y, and just over half, 50.8%, were double-positive for both, a level of granularity a bulk assay simply cannot deliver.

Fig. 3.  Top: EV samples vs. antibody-only controls for protein X and protein Y individually. Bottom: single- and double-staining of EVs resolves four subpopulations, from double-negative (27.7%) through to double-positive (50.8%).

Cracking Open the LNP

The third application note shifts from biological messengers to synthetic drug delivery, asking a pointed question: for a batch of mRNA-loaded LNPs, how much mRNA is actually sealed inside, versus just stuck on the outside? The logic is elegant. LNPs were dual-stained, a membrane dye (DiD) marking every particle, and an RNA-binding dye (SYTO RNA Select Green) marking particles carrying nucleic acid, then challenged with RNase, an enzyme that chews up exposed RNA but can't reach RNA genuinely sealed inside.

Fig. 4.  DiD vs. SYTO RNA Select Green dot plots for (A) an mRNA-free LNP baseline, (B) mRNA properly encapsulated during LNP formation, and (C) mRNA simply mixed with pre-formed LNPs, before and after RNase challenge.

For LNPs manufactured with mRNA encapsulated during formation, 93.1% of particles were RNA-positive before RNase treatment; after digestion, that only slipped to 74.9%, as most cargo was protected, as intended.

Contrast that with mRNA simply mixed with pre-formed LNPs: 74.4% appeared RNA-positive before RNase, collapsing to just 7.9% afterward. Nearly all of that previous signal was actually surface-bound mRNA sitting in harm's way, invisible to a bulk assay, but obvious once you can watch individual particles survive or fail an enzymatic stress test.

The Bigger Picture

These assays depend on instrumentation sensitive enough to resolve subcellular particles at useful throughput. The ZE5 Cell Analyzer platform supports up to 100,000 events per second, with noise reduction to separate true signal from background, while its temperature-controlled loader (4–37°C) helps track formulation or storage stability.

Together, these application notes show single-particle characterization becoming a practical tool for microbiology, EV biology, and nanoparticle drug development, replacing bulk averages with particle-level answers.