Bold selected part of my string in a text view

from the UITextView documentation on the font property:

font The font of the text.

@property(nonatomic, retain) UIFont *font Discussion This property applies to the entire text string. The default font is a 17-point Helvetica plain font.

In iOS 6 and later, assigning a new value to this property causes the new font to be applied to the entire contents of the text view. If you want to apply the font to only a portion of the text, you must create a new attributed string with the desired style information and assign it to the attributedText property.

so perhaps try assigning that string to the attributedText property:

 NSString *boldFontName = [[UIFont boldSystemFontOfSize:12] fontName];
    NSString *yourString = textView.text;
    NSRange boldedRange = [textView textInRange:selectedRange];

    NSMutableAttributedString *attrString = [[NSMutableAttributedString alloc] initWithString:yourString];

    [attrString beginEditing];
    [attrString addAttribute:kCTFontAttributeName
                       value:boldFontName
                       range:boldedRange];

    [attrString endEditing];

   texView.attributedText = attrString;