Two-dimensional materials like graphene and MXenes are no longer just “cool nanostuff” for physics labs—they’re starting to look very interesting for biomedical applications, too. But there’s a big catch:

A material can be fantastic on paper and completely useless in the body if we don’t control its surface – its charge, stability in liquids, and how it interacts with cells and bacteria.

The article we’re summarizing looks exactly at this problem for one specific MXene: Ti₃C₂, a titanium carbide MXene. The researchers asked a simple but powerful question:

If we deliberately change the surface charge of Ti₃C₂ MXene using a positively charged polymer, can we improve its antibacterial properties while keeping it safe for mammalian cells?

They did this by coating Ti₃C₂ flakes with poly-L-lysine (PLL), a positively charged, biologically relevant polymer. Then they carefully studied:

• How this coating changes surface charge and colloidal behavior

• How it affects bacteria (E. coli)

• How it affects human skin cells (normal and cancerous)

Let’s walk through what they did and what they found, in clear, blog-style language.

1. Background: MXenes, Ti₃C₂, and Why Surface Matters

What are MXenes?

MXenes are a family of 2D materials made from carbides, nitrides or carbonitrides of transition metals. Their general formula is:

Mₙ₊₁XₙTₓ

Where:

• M is a transition metal (like Ti, Nb, V, Mo)

• X is carbon and/or nitrogen

• Tₓ are surface terminations such as –OH, =O, or –F

They are typically made by etching one element (usually Al) out of a layered ceramic called a MAX phase. This creates thin, sheet-like structures—kind of like a stack of cards where some cards have been removed and the rest can slide apart into individual sheets.

Why Ti₃C₂?

Among many MXenes (like Ti₂C, Nb₂C, V₂C, etc.), Ti₃C₂ is one of the best known and most widely used because:

• It’s relatively easy to synthesize reproducibly

• It’s already been explored for energy storage, composites, and pollutant adsorption

• It’s commercially available and well-studied compared to other MXenes

More recently, MXenes, including Ti₃C₂, have attracted attention in biomedicine and nanomedicine for:

• Biosensing

• Photothermal therapy

• Drug delivery

• Antibacterial coatings or barriers

But there are still big open questions:

• How toxic are they to mammalian cells?

• How do they interact with bacteria?

• How can we control their surface in a simple, reliable way?

All of this is tightly connected to surface charge, stability in water, and interaction with biomolecules.

Why surface charge is such a big deal

Ti₃C₂ MXene naturally has a negative surface charge in water (because of its surface terminations). This can strongly influence:

• How it disperses or aggregates (colloidal stability)

• How it interacts with negatively charged bacterial cells

• How it behaves in physiological environments

If we can systematically tune this charge—without destroying the material—we can “steer” its biological effects.

2. The Strategy: Coating Ti₃C₂ with Poly-L-Lysine (PLL)

What is PLL?

Poly-L-lysine (PLL) is a cationic (positively charged) polymer made from the amino acid lysine. It’s already used in biotechnology, for example to:

• Improve cell adhesion on culture surfaces

• Act as an antibacterial agent

• Provide a high positive charge thanks to its amine groups

Because Ti₃C₂ is negatively charged, and PLL is positively charged, the researchers used this simple principle:

Opposites attract → PLL should adsorb onto the Ti₃C₂ surface via electrostatic interactions.

This would:

• Flip the surface charge from negative to positive

• Potentially improve interaction with bacteria (whose membranes are typically negatively charged)

• Possibly modify toxicity toward mammalian cells

The resulting hybrid is referred to as Ti₃C₂/PLL or “2D Ti₃C₂ flakes surface-modified with PLL”.

3. How the Material Was Made and Characterized (Without Overcomplicating It)

From MAX phase to 2D Ti₃C₂ flakes

1. They first synthesized the Ti₃AlC₂ MAX phase using:

   • Titanium, aluminum, and graphite powders

   • Spark Plasma Sintering (SPS), a high-temperature, fast sintering method

   • Reaction at 1300 °C to form Ti₃AlC₂

2. Then they etched out aluminum (Al) from Ti₃AlC₂ using 48% hydrofluoric acid (HF):

   • MXene powder was immersed in HF

   • Reaction time: 24 hours at room temperature

   • This removes Al and leaves behind Ti₃C₂ layers with slit-like pores where Al used to be.

3. The material was washed, dried, and then delaminated:

   • Delamination used tetramethylammonium hydroxide (TMAOH) in water to help separate the layers into thinner flakes.

   • Mild sonication and centrifugation produced stable colloidal dispersions of Ti₃C₂.

   • Finally, the flakes were freeze-dried to obtain a powdered form.

What do the Ti₃C₂ flakes look like?

Using electron microscopes, the researchers observed:

• The dried material forms loose agglomerates of many flakes.

• Individual flakes:

  • Have irregular shapes with sharp edges

  • Show clearly layered structures, typical of 2D materials

• At very high magnification, they could even resolve the individual layers (Ti–C–Ti–C–Ti sequence) and measure the distance between them:

  • Interlayer spacing was about 1.21 nm, slightly increased due to delamination and intercalation.

Elemental analysis showed the expected elements:

• Titanium (Ti) and carbon (C) from the MXene backbone

• Oxygen (O) and fluorine (F) from surface terminations formed during etching

• Traces of chlorine (from pH adjustment) and copper (from the sample holder)

How porous is the material?

They measured nitrogen gas adsorption to determine:

• Total specific surface area: about 93 m²/g

• External surface area: ~61.5 m²/g

• Micropore surface area: ~31.5 m²/g (about 34% of total)

This means the delaminated Ti₃C₂ MXene has roughly five times higher surface area than etched but not delaminated material. More surface area usually means:

• More sites for adsorption

• More potential interaction points with cells and bacteria

4. Engineering the Surface Charge with PLL

This is the heart of the study: how PLL changes the surface charge and colloidal behavior of Ti₃C₂.

Step 1 – Measuring zeta potential: how charged is the surface?

They used zeta potential measurements to quantify surface charge:

• Pristine Ti₃C₂ flakes in water:

  • Zeta potential ≈ −5.6 mV (slightly negative)

• After adding PLL step by step (MXene:PLL ratios from 1:0 up to 1:20):

  • The zeta potential gradually increased from negative to strongly positive

  • At relatively low PLL amounts (around 1:1 ratio), the surface charge flipped to positive

  • It eventually reached around +40 to +45 mV, indicating strongly positive, PLL-covered surfaces

What does this mean?

• PLL adsorbs efficiently onto MXene flakes.

• A monolayer of PLL forms quickly and is enough to flip the surface charge.

• With more PLL, multilayer coverage is likely.

Final result:

• Ti₃C₂: mildly negative surface

• Ti₃C₂/PLL: strongly positive surface (~+45 mV)

This is exactly what the researchers wanted: surface charge engineering.

Step 2 – Looking at flake size and agglomeration (DLS measurements)

They also measured hydrodynamic diameter (how big the flakes or clusters look in water) using dynamic light scattering (DLS):

• For pristine Ti₃C₂:

  • Flakes and floccules (clusters) showed sizes from ~100 nm up to hundreds of nm or more.

  • The system contains both single flakes and agglomerates.

• After modification with PLL (e.g., 1:1 ratio):

  • Single flake hydrodynamic diameter increased to around 350 nm, indicating some flocculation.

  • However, this clustering was reversible: gentle shaking could break them back into smaller units.

When they added PLL gradually:

• At low PLL amounts (1:1), the hydrodynamic diameter of flakes actually decreased sharply (from about 800 nm to ~100 nm), suggesting deflocculation and improved dispersion.

• At higher PLL ratios, they saw a “plateau” in size (100–300 nm), and then, beyond a certain point, sizes increased again (stronger flocculation as more PLL is added).

In simple terms:

• A moderate amount of PLL helps to stabilize and disperse the flakes.

• Too much PLL can cause them to stick together more.

• Overall, PLL allows finer control over colloidal stability.

Step 3 – How does pH affect charge and stability?

The team also checked how pH changes the zeta potential and particle size:

• Pristine Ti₃C₂:

  • Zeta potential becomes more negative as pH increases (from about −8 mV at pH ~4 to around −22 mV at pH ~11).

  • This means better electrostatic stabilization at higher pH.

• Ti₃C₂/PLL:

  • At lower pH (around 3.5–8.5), the surface remains strongly positive, but zeta potential slowly decreases as pH increases.

  • Around pH ~10.6, the surface passes through an isoelectric point (zeta potential near zero) and then becomes negative again.

  • This behavior suggests that PLL starts to desorb from the surface at high pH, leaving behind negatively charged MXene.

At high pH:

• Hydrodynamic diameters of Ti₃C₂/PLL decrease (down toward ~100 nm).

• This means the system deflocculates and becomes more stable in dispersion.

This reversible desorption of PLL with pH is very interesting:

It hints at potential pH-responsive behavior, which could be useful for drug delivery or controlled release applications.

5. Confirming PLL Is Really on the Surface (FTIR)

To make sure PLL is truly attached to the MXene surface, they used FTIR (infrared spectroscopy):

• They recorded spectra for:

  • Pure PLL

  • Pure Ti₃C₂

  • Ti₃C₂/PLL hybrid

The hybrid’s spectrum shows:

• Bands associated with PLL (e.g., from C–NH₂ and N–H groups)

• Bands associated with Ti₃C₂ MXene (e.g., C–F and C–H)

• Shared bands such as C=O

Importantly:

• There were no new, unexpected bands suggesting chemical degradation.

• Both PLL and Ti₃C₂ appear intact in the hybrid.

This supports the idea that PLL is attached mainly via electrostatic interactions, not by chemically breaking or altering the MXene or polymer.

6. Antibacterial Properties: Fighting E. coli

Next, the researchers tested whether these surface changes led to different antibacterial behavior.

What did they test?

They used E. coli MG1655, a common Gram-negative model bacterium, and incubated the bacteria with:

• Unmodified Ti₃C₂ flakes

• Ti₃C₂/PLL flakes

They carefully removed unbound PLL from the MXene/PLL mixture, so the observed effects come mainly from the hybrid material, not free PLL in solution.

Results:

• Unmodified Ti₃C₂:

  • Showed no significant antibacterial activity under the test conditions.

  • This aligns with its moderate negative charge and tendency to form aggregates, which likely reduces direct contact with bacterial cell surfaces.

• Ti₃C₂/PLL:

  • Showed clear antibacterial activity.

  • After 6 hours of co-incubation at a concentration around 200 mg/L, the number of viable E. coli cells dropped by about two orders of magnitude (i.e., ~100× reduction).

  • Below that concentration, no strong antibacterial effect was observed.

  • Complete sterilization (zero viable cells) was not achieved, even at higher concentrations.

Why does Ti₃C₂/PLL work better?

• Bacteria often have negatively charged cell walls.

• Positively charged surfaces (like MXene coated with PLL) can:

  • Attach more strongly to bacterial membranes

  • Disrupt membrane integrity

  • Interfere with respiration or metabolism

The exact molecular mechanism still needs more research, but it’s clear that surface charge and good dispersion are essential for antibacterial effects in this system.

7. Cytotoxicity: Is It Safe for Human Cells?

Antibacterial action is great—but not if the material is also highly toxic to human cells. So the team tested in vitro cytotoxicity using two human skin cell lines:

• HaCaT – immortalized, but non-cancerous human keratinocytes (normal-like skin cells)

• A375 – human malignant melanoma cells (cancer cells)

They used the MTT assay, which measures metabolic activity as a proxy for cell viability, after 24 hours of exposure to:

• Pristine Ti₃C₂

• Ti₃C₂/PLL

Both across a wide concentration range (up to 375–500 mg/L).

Key findings:

• In the concentration range up to about 375 mg/L, both forms (with and without PLL) showed no significant cytotoxicity toward HaCaT or A375 cells.

• In other words, the cells remained largely viable at these doses.

• At the highest tested concentrations (around 500 mg/L), they saw more noticeable toxicity, and they further analyzed this using cell cycle analysis.

Cell cycle results (mechanistic hint):

• For HaCaT cells:

  • Pristine Ti₃C₂ did not significantly change the cell cycle profile compared to control.

  • Ti₃C₂/PLL caused a G0/G1 phase arrest, which is often associated with the induction of apoptosis (programmed cell death).

• For A375 melanoma cells:

  • Both pristine and PLL-modified Ti₃C₂ showed signs of apoptosis-related effects in the cell cycle distribution.

This is interesting because:

• It suggests that at higher doses, these materials might trigger programmed cell death, especially in cancer cells—something potentially useful for anticancer strategies.

• However, in the lower to moderate concentration range, they do not show strong toxicity, which is important for safety.

8. What Does All This Mean?

Putting it all together, this study shows a practical and biologically relevant strategy for tailoring MXenes:

1. Surface charge control via PLL

   • Simple adsorption of PLL flips the Ti₃C₂ surface from negative to strongly positive.

   • The process is tunable via PLL amount and pH.

   • PLL can even be partially desorbed by increasing pH, hinting at controlled release possibilities.

2. Improved colloidal behavior

   • PLL helps adjust the balance between dispersion and flocculation.

   • At suitable ratios, the MXene disperses better in water, which is critical for biological testing and applications.

3. Enhanced antibacterial activity

   • Unmodified Ti₃C₂ showed no real effect on E. coli.

   • Ti₃C₂/PLL significantly reduced viable E. coli counts at sufficiently high concentrations, thanks to surface charge and better interaction.

4. Low cytotoxicity window for mammalian cells

   • Up to around 375 mg/L, both Ti₃C₂ and Ti₃C₂/PLL show limited or no substantial toxicity toward skin cells.

   • Higher doses begin to involve mechanisms like apoptosis, especially in cancer cells.

Why is this important?

For biotechnology and nanomedicine, any candidate material must meet several conditions at once:

• Be dispersible and stable in aqueous/physiological environments

• Have controllable surface properties (especially charge)

• Offer useful biological functions (e.g., antibacterial action)

• Be reasonably safe for mammalian cells at relevant concentrations

This work shows that:

By simple surface charge engineering with poly-L-lysine, 2D Ti₃C₂ MXene flakes can be transformed into a hybrid nanomaterial with improved antibacterial activity and acceptable biocompatibility, while also having tunable colloidal properties.

It doesn’t solve all open questions, but it provides a feasible, experimentally validated pathway for integrating MXenes into future biomedical and biotechnological applications, such as antimicrobial coatings, smart delivery systems, or biointerfaces.